/* =========================================================
   StudyScope — lesson design system (tokens, lesson layout, shared components)
   ========================================================= */

:root {
  /* Colors */
  --color-bg: #f4f5f9;
  --color-surface: #ffffff;
  --color-surface-muted: #f8f9fc;
  --color-border: #e2e5ee;
  --color-border-strong: #cfd4e2;
  --color-text: #15203b;
  --color-text-muted: #56607a;
  --color-accent: #5046d6;
  --color-accent-strong: #3d34b3;
  --color-accent-soft: #f0eeff;
  --color-success: #1e9467;
  --color-success-strong: #146c4a;
  --color-success-soft: #e7f6ef;
  --color-danger: #d4434f;
  --color-danger-strong: #aa2b36;
  --color-danger-soft: #fdecee;
  --color-warning-soft: #fff6e0;

  /* Graph colors (read by script.js) */
  --graph-grid: #eceef4;
  --graph-axis: #8a92a8;
  --graph-inc: #2e9e74;
  --graph-dec: #d9606b;
  --graph-inc-soft: rgba(46, 158, 116, 0.11);
  --graph-dec-soft: rgba(217, 96, 107, 0.11);
  --graph-alt: #c27a2c;
  --graph-tangent: #1e9467;

  /* Typography */
  --font-sans: "Inter", "Segoe UI", system-ui, -apple-system, "Helvetica Neue", Arial, sans-serif;
  --font-math: "Cambria Math", "Cambria", "STIX Two Math", "Times New Roman", Georgia, serif;
  --text-xs: 0.8125rem;
  --text-sm: 0.9375rem;
  --text-md: 1.0625rem;
  --text-lg: 1.25rem;
  --text-xl: clamp(1.6rem, 1.2rem + 1.6vw, 2.25rem);
  --text-hero: clamp(1.5rem, 1.2rem + 1.2vw, 2rem);

  /* Spacing */
  --space-1: 0.25rem;
  --space-2: 0.5rem;
  --space-3: 0.75rem;
  --space-4: 1rem;
  --space-5: 1.5rem;
  --space-6: 2rem;
  --space-7: 3rem;

  /* Radius / shadow */
  --radius-sm: 8px;
  --radius-md: 12px;
  --radius-lg: 18px;
  --radius-pill: 999px;
  --shadow-sm: 0 1px 2px rgba(21, 32, 59, 0.06);
  --shadow-md: 0 1px 2px rgba(21, 32, 59, 0.05), 0 8px 24px rgba(21, 32, 59, 0.07);

  /* Motion */
  --ease: cubic-bezier(0.2, 0.7, 0.2, 1);
  --dur-fast: 150ms;
  --dur-med: 320ms;
  --dur-slow: 520ms;

  --actionbar-height: 76px;
}

/* ---------- Base ---------- */

*,
*::before,
*::after { box-sizing: border-box; }

html { -webkit-text-size-adjust: 100%; }

/* `hidden` must win over component display rules (.btn, .keypoint, .score … set display). */
[hidden] { display: none !important; }

body {
  margin: 0;
  background: var(--color-bg);
  color: var(--color-text);
  font-family: var(--font-sans);
  font-size: var(--text-md);
  line-height: 1.6;
  overflow-x: hidden;
}

h1, h2, h3, p { margin: 0; }

p + p { margin-top: var(--space-3); }

button { font: inherit; color: inherit; }

strong { font-weight: 650; }

.math {
  font-family: var(--font-math);
  font-size: 1.08em;
  white-space: nowrap;
}
.math i { font-style: italic; padding-right: 0.09em; }

.visually-hidden {
  position: absolute !important;
  width: 1px; height: 1px;
  margin: -1px; padding: 0;
  overflow: hidden;
  clip: rect(0 0 0 0);
  white-space: nowrap;
  border: 0;
}

.skip-link {
  position: absolute;
  left: var(--space-4);
  top: -100px;
  z-index: 100;
  padding: var(--space-2) var(--space-4);
  background: var(--color-text);
  color: #fff;
  border-radius: var(--radius-sm);
}
.skip-link:focus { top: var(--space-3); }

:focus-visible {
  outline: 3px solid var(--color-accent);
  outline-offset: 3px;
  border-radius: var(--radius-sm);
}

/* ---------- Top bar ---------- */

.topbar {
  position: sticky;
  top: 0;
  z-index: 20;
  background: rgba(244, 245, 249, 0.88);
  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);
  border-bottom: 1px solid var(--color-border);
}

.topbar__inner {
  max-width: 1180px;
  margin: 0 auto;
  padding: var(--space-3) var(--space-5);
  display: flex;
  align-items: center;
  gap: var(--space-5);
}

.brand {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  flex-shrink: 0;
}

.brand__mark {
  display: grid;
  place-items: center;
  width: 38px;
  height: 38px;
  border-radius: 11px;
  background: var(--color-accent);
  color: #fff;
  font-family: var(--font-math);
  font-style: italic;
  font-size: 1.2rem;
  font-weight: 600;
  letter-spacing: 0.12em;
  padding-left: 0.12em;
}

.brand__eyebrow {
  font-size: var(--text-xs);
  color: var(--color-text-muted);
  line-height: 1.2;
}

.brand__title {
  font-weight: 650;
  line-height: 1.3;
}

.progress {
  flex: 1;
  min-width: 0;
  display: flex;
  align-items: center;
  gap: var(--space-4);
}

.progress__label {
  order: 2;
  font-size: var(--text-sm);
  font-weight: 600;
  color: var(--color-text-muted);
  white-space: nowrap;
  font-variant-numeric: tabular-nums;
}

.progress__track {
  flex: 1;
  height: 8px;
  border-radius: var(--radius-pill);
  background: #e3e6f0;
  overflow: hidden;
}

.progress__fill {
  width: 0%;
  height: 100%;
  border-radius: inherit;
  background: linear-gradient(90deg, var(--color-accent), #6a55ee);
  transition: width var(--dur-slow) var(--ease);
}

/* ---------- Lesson layout ---------- */

.lesson {
  max-width: 1180px;
  margin: 0 auto;
  padding: var(--space-6) var(--space-5) calc(var(--actionbar-height) + var(--space-7));
}

.stage { display: none; }

.stage.is-active {
  display: grid;
  grid-template-columns: minmax(0, 0.95fr) minmax(0, 1.05fr);
  gap: var(--space-7);
  align-items: start;
  animation: stage-in var(--dur-slow) var(--ease) both;
}

.stage--summary.is-active {
  grid-template-columns: minmax(0, 720px);
  justify-content: center;
}

@keyframes stage-in {
  from { opacity: 0; transform: translateY(14px); }
  to   { opacity: 1; transform: none; }
}

.stage__text { padding-top: var(--space-4); }

.stage__text > * + * { margin-top: var(--space-4); }

.eyebrow {
  font-size: var(--text-xs);
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--color-accent);
}

.stage__title {
  font-size: var(--text-xl);
  line-height: 1.2;
  font-weight: 700;
  letter-spacing: -0.01em;
  margin-top: var(--space-2);
}
.stage__title:focus { outline: none; }

.lead {
  font-size: var(--text-lg);
  line-height: 1.5;
  font-weight: 500;
}

.stage__text p:not(.eyebrow):not(.lead) { color: var(--color-text-muted); }
.stage__text strong { color: var(--color-text); }

.formula-line {
  display: flex;
  flex-wrap: wrap;
  align-items: baseline;
  gap: var(--space-3);
  font-size: var(--text-lg);
}
.stage__text .formula-line { color: var(--color-text); }
.formula-line__sep { color: var(--color-border-strong); }

.formula-hero {
  display: inline-block;
  padding: var(--space-3) var(--space-5);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  box-shadow: var(--shadow-sm);
  font-size: var(--text-hero);
}
.stage__text .formula-hero { color: var(--color-text); }

.footnote {
  font-size: var(--text-xs);
  line-height: 1.55;
  color: var(--color-text-muted);
}

.tip {
  display: flex;
  gap: var(--space-3);
  padding: var(--space-3) var(--space-4);
  background: var(--color-accent-soft);
  border-radius: var(--radius-md);
  font-size: var(--text-sm);
}
.stage__text .tip { color: var(--color-text); }

.plan {
  padding: var(--space-4) var(--space-5);
  border: 1px dashed var(--color-border-strong);
  border-radius: var(--radius-md);
}
.plan__title { font-weight: 600; font-size: var(--text-sm); }
.plan__list {
  margin: var(--space-2) 0 0;
  padding-left: var(--space-5);
  color: var(--color-text-muted);
  font-size: var(--text-sm);
}

/* ---------- Cards ---------- */

.card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-md);
  padding: var(--space-5);
}

.card__label {
  font-size: var(--text-xs);
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--color-text-muted);
  margin-bottom: var(--space-4);
}

/* ---------- Rule rows ---------- */

.rule-card { padding: var(--space-6); }
.rule-card .footnote { margin-top: var(--space-5); }

.rule {
  display: grid;
  grid-template-columns: auto auto 1fr;
  align-items: center;
  gap: var(--space-4);
  padding: var(--space-4) var(--space-5);
  border-radius: var(--radius-md);
  border: 1px solid transparent;
}
.rule + .rule { margin-top: var(--space-3); }

.rule--inc { background: var(--color-success-soft); border-color: rgba(30, 148, 103, 0.18); }
.rule--dec { background: var(--color-danger-soft); border-color: rgba(212, 67, 79, 0.18); }
.rule--zero { background: var(--color-accent-soft); border-color: rgba(80, 70, 214, 0.18); }

.rule__condition {
  font-size: var(--text-hero);
  font-weight: 500;
}
.rule__arrow {
  font-size: 1.4rem;
  color: var(--color-text-muted);
}
.rule__result {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  flex-wrap: wrap;
  line-height: 1.35;
}
.rule--inc .rule__result strong,
.rule--inc .trend-icon { color: var(--color-success-strong); }
.rule--dec .rule__result strong,
.rule--dec .trend-icon { color: var(--color-danger-strong); }
.rule--zero .trend-icon { color: var(--color-accent); }

.trend-icon {
  display: inline-grid;
  place-items: center;
  width: 30px; height: 30px;
  border-radius: 50%;
  background: rgba(255, 255, 255, 0.8);
  font-weight: 700;
  flex-shrink: 0;
}

/* ---------- Graph ---------- */

.graph {
  display: block;
  width: 100%;
  aspect-ratio: 4 / 3;
  border-radius: var(--radius-md);
  background: var(--color-surface-muted);
  touch-action: pan-y;
}
.graph--short { aspect-ratio: 16 / 10; }

@media (min-width: 961px) {
  .graph-card .graph { aspect-ratio: 16 / 11; }
}

.graph-card { display: flex; flex-direction: column; gap: var(--space-3); }

.legend {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: var(--space-2) var(--space-5);
  font-size: var(--text-xs);
  color: var(--color-text-muted);
}
.legend__item { display: inline-flex; align-items: center; gap: var(--space-2); }
.legend__item::before {
  content: "";
  width: 18px; height: 4px;
  border-radius: 2px;
}
.legend__item--dec::before { background: var(--graph-dec); }
.legend__item--inc::before { background: var(--graph-inc); }

.graph-caption {
  margin-top: var(--space-3);
  font-size: var(--text-sm);
  color: var(--color-text-muted);
  line-height: 1.55;
}

/* ---------- Slider ---------- */

.slider-field { display: grid; gap: var(--space-3); }

.slider-field__label {
  font-size: var(--text-sm);
  font-weight: 600;
}
.slider-field__label output {
  display: inline-block;
  min-width: 2.5em;
  color: var(--color-accent-strong);
}

.slider {
  --fill: 25%;
  width: 100%;
  height: 32px;
  margin: 0;
  background: transparent;
  -webkit-appearance: none;
  appearance: none;
  cursor: pointer;
}
.slider::-webkit-slider-runnable-track {
  height: 6px;
  border-radius: var(--radius-pill);
  background: linear-gradient(90deg, var(--color-accent) var(--fill), #dfe2ee var(--fill));
}
.slider::-moz-range-track {
  height: 6px;
  border-radius: var(--radius-pill);
  background: #dfe2ee;
}
.slider::-moz-range-progress {
  height: 6px;
  border-radius: var(--radius-pill);
  background: var(--color-accent);
}
.slider::-webkit-slider-thumb {
  -webkit-appearance: none;
  width: 26px; height: 26px;
  margin-top: -10px;
  border-radius: 50%;
  background: #fff;
  border: 3px solid var(--color-accent);
  box-shadow: 0 2px 6px rgba(21, 32, 59, 0.2);
  transition: transform var(--dur-fast) var(--ease);
}
.slider::-moz-range-thumb {
  width: 20px; height: 20px;
  border-radius: 50%;
  background: #fff;
  border: 3px solid var(--color-accent);
  box-shadow: 0 2px 6px rgba(21, 32, 59, 0.2);
}
.slider:active::-webkit-slider-thumb { transform: scale(1.12); }
.slider:focus-visible { outline-offset: 2px; }

.quick-picks {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-2);
}

.chip {
  min-width: 48px;
  min-height: 40px;
  padding: 0 var(--space-3);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-pill);
  background: var(--color-surface);
  font-family: var(--font-math);
  font-size: 1rem;
  cursor: pointer;
  transition: background var(--dur-fast), border-color var(--dur-fast);
}
.chip:hover { border-color: var(--color-accent); background: var(--color-accent-soft); }

/* ---------- Readout ---------- */

.readout {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: var(--space-3);
  margin: 0;
}

.readout__cell {
  padding: var(--space-3) var(--space-4);
  border-radius: var(--radius-md);
  background: var(--color-surface-muted);
  border: 1px solid var(--color-border);
  transition: background var(--dur-med), border-color var(--dur-med);
}
.readout__cell dt {
  font-size: var(--text-sm);
  color: var(--color-text-muted);
}
.readout__cell dd {
  margin: 0;
  font-size: 1.5rem;
  font-weight: 600;
  font-variant-numeric: tabular-nums;
}

.readout__cell--deriv[data-sign="neg"] { background: var(--color-danger-soft); border-color: rgba(212, 67, 79, 0.3); }
.readout__cell--deriv[data-sign="neg"] dd { color: var(--color-danger-strong); }
.readout__cell--deriv[data-sign="pos"] { background: var(--color-success-soft); border-color: rgba(30, 148, 103, 0.3); }
.readout__cell--deriv[data-sign="pos"] dd { color: var(--color-success-strong); }
.readout__cell--deriv[data-sign="zero"] { background: var(--color-accent-soft); border-color: rgba(80, 70, 214, 0.3); }
.readout__cell--deriv[data-sign="zero"] dd { color: var(--color-accent-strong); }

/* ---------- Insight message ---------- */

.insight {
  padding: var(--space-4) var(--space-5);
  border-radius: var(--radius-md);
  border-left: 4px solid var(--color-border-strong);
  background: var(--color-surface-muted);
  transition: border-color var(--dur-med), background var(--dur-med);
}
.insight__title { font-weight: 650; }
.insight__body { margin-top: var(--space-1); color: var(--color-text-muted); font-size: var(--text-sm); }
.insight[data-sign="neg"] { border-color: var(--color-danger); background: var(--color-danger-soft); }
.insight[data-sign="pos"] { border-color: var(--color-success); background: var(--color-success-soft); }
.insight[data-sign="zero"] { border-color: var(--color-accent); background: var(--color-accent-soft); }
.insight.is-updated { animation: fade-in var(--dur-med) var(--ease); }

/* ---------- Checklist ---------- */

.checklist {
  padding: var(--space-4) var(--space-5);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
}
.checklist__title { font-weight: 650; font-size: var(--text-sm); }
.stage__text .checklist__title { color: var(--color-text); }
.checklist__list {
  list-style: none;
  margin: var(--space-2) 0 0;
  padding: 0;
  display: grid;
  gap: var(--space-2);
}
.checklist__item {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  font-size: var(--text-sm);
  color: var(--color-text-muted);
  transition: color var(--dur-med);
}
.checklist__box {
  display: grid;
  place-items: center;
  width: 22px; height: 22px;
  border-radius: 7px;
  border: 2px solid var(--color-border-strong);
  flex-shrink: 0;
  transition: background var(--dur-med), border-color var(--dur-med);
}
.checklist__item.is-done { color: var(--color-text); }
.checklist__item.is-done .checklist__box {
  background: var(--color-success);
  border-color: var(--color-success);
}
.checklist__item.is-done .checklist__box::after {
  content: "";
  width: 10px; height: 6px;
  margin-top: -2px;
  border-left: 2.5px solid #fff;
  border-bottom: 2.5px solid #fff;
  transform: rotate(-45deg);
  animation: pop-in var(--dur-med) var(--ease);
}

/* ---------- Quiz / options ---------- */

.quiz__question {
  font-size: var(--text-lg);
  line-height: 1.45;
  font-weight: 650;
  margin-bottom: var(--space-5);
}

.options { display: grid; gap: var(--space-3); }

.option {
  display: flex;
  align-items: center;
  gap: var(--space-4);
  width: 100%;
  min-height: 60px;
  padding: var(--space-3) var(--space-4);
  text-align: left;
  background: var(--color-surface);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  cursor: pointer;
  transition: border-color var(--dur-fast), background var(--dur-fast), transform var(--dur-fast), opacity var(--dur-med);
}
.option:hover:not(:disabled) {
  border-color: var(--color-accent);
  background: var(--color-accent-soft);
}
.option:active:not(:disabled) { transform: scale(0.99); }
.option:disabled { cursor: default; }

.option__key {
  display: grid;
  place-items: center;
  width: 34px; height: 34px;
  border-radius: 10px;
  background: var(--color-surface-muted);
  border: 1px solid var(--color-border);
  font-weight: 700;
  font-size: var(--text-sm);
  color: var(--color-text-muted);
  flex-shrink: 0;
  transition: background var(--dur-fast), color var(--dur-fast), border-color var(--dur-fast);
}
.option__text { flex: 1; font-weight: 550; }
.option__status {
  width: 26px; height: 26px;
  flex-shrink: 0;
  display: grid;
  place-items: center;
  border-radius: 50%;
  font-weight: 700;
  font-size: 0.9rem;
}

.option.is-correct {
  border-color: var(--color-success);
  background: var(--color-success-soft);
  animation: pop-in var(--dur-med) var(--ease);
}
.option.is-correct .option__key { background: var(--color-success); border-color: var(--color-success); color: #fff; }
.option.is-correct .option__status { background: var(--color-success); color: #fff; }
.option.is-correct .option__status::before { content: "✓"; }

.option.is-wrong {
  border-color: var(--color-danger);
  background: var(--color-danger-soft);
  animation: nudge 360ms var(--ease);
}
.option.is-wrong .option__key { background: var(--color-danger); border-color: var(--color-danger); color: #fff; }
.option.is-wrong .option__status { background: var(--color-danger); color: #fff; }
.option.is-wrong .option__status::before { content: "✕"; }

.options.is-solved .option:not(.is-correct) { opacity: 0.5; }

/* ---------- Feedback ---------- */

.feedback:empty { display: none; }

.feedback {
  display: flex;
  gap: var(--space-3);
  margin-top: var(--space-4);
  padding: var(--space-4);
  border-radius: var(--radius-md);
  border: 2px solid transparent;
  animation: fade-in var(--dur-med) var(--ease);
}
.feedback__icon {
  width: 28px; height: 28px;
  flex-shrink: 0;
  display: grid;
  place-items: center;
  border-radius: 50%;
  color: #fff;
}
.feedback__title { font-weight: 650; }
.feedback__body { margin-top: var(--space-1); font-size: var(--text-sm); color: var(--color-text-muted); }
.feedback__body + .feedback__body { margin-top: var(--space-1); }

.feedback--success { background: var(--color-success-soft); border-color: rgba(30, 148, 103, 0.35); }
.feedback--success .feedback__icon { background: var(--color-success); }
.feedback--success .feedback__title { color: var(--color-success-strong); }

.feedback--error { background: var(--color-danger-soft); border-color: rgba(212, 67, 79, 0.35); }
.feedback--error .feedback__icon { background: var(--color-danger); }
.feedback--error .feedback__title { color: var(--color-danger-strong); }

.feedback--info { background: var(--color-warning-soft); border-color: rgba(196, 140, 20, 0.3); }
.feedback--info .feedback__icon { background: #b7800f; }

/* ---------- Solution reveal ---------- */

.solution {
  margin-top: var(--space-5);
  padding-top: var(--space-5);
  border-top: 1px solid var(--color-border);
  animation: fade-in var(--dur-slow) var(--ease);
}
.solution__title {
  font-size: var(--text-xs);
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--color-text-muted);
  margin-bottom: var(--space-3);
}
.solution__steps {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: var(--space-2);
  margin: 0 0 var(--space-4);
  padding: 0;
  list-style: none;
  font-size: var(--text-lg);
  white-space: normal;
}
.solution__steps li {
  padding: var(--space-1) var(--space-3);
  background: var(--color-surface-muted);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
}
.solution__steps li + li::before {
  content: "⇔";
  margin-right: var(--space-3);
  margin-left: calc(-1 * var(--space-2));
  color: var(--color-text-muted);
  font-style: normal;
}
.solution__steps li:last-child {
  background: var(--color-success-soft);
  border-color: rgba(30, 148, 103, 0.35);
  color: var(--color-success-strong);
  font-weight: 600;
}

/* ---------- Sign table (stage 4) ---------- */

.table-wrap {
  overflow-x: auto;
  margin-bottom: var(--space-5);
}

.sign-table {
  width: 100%;
  border-collapse: separate;
  border-spacing: 0;
  text-align: center;
  font-size: var(--text-md);
}
.sign-table th,
.sign-table td {
  padding: var(--space-3) var(--space-2);
  border-bottom: 1px solid var(--color-border);
  transition: background var(--dur-med) var(--ease);
}
.sign-table tr:last-child th,
.sign-table tr:last-child td { border-bottom: 0; }
.sign-table th {
  width: 18%;
  text-align: left;
  padding-left: var(--space-3);
  font-weight: 500;
  border-right: 1px solid var(--color-border);
}
.sign-table__critical { color: var(--color-accent-strong); font-weight: 600; }

.sign-table__row { opacity: 0; transform: translateY(8px); }
.sign-table.is-revealed .sign-table__row {
  animation: row-in var(--dur-slow) var(--ease) forwards;
  animation-delay: calc(var(--i) * 220ms + 120ms);
}

@keyframes row-in {
  to { opacity: 1; transform: none; }
}

.sign {
  display: inline-grid;
  place-items: center;
  min-width: 34px; height: 34px;
  border-radius: 50%;
  font-weight: 700;
  font-size: 1.2rem;
}
.sign--neg { background: var(--color-danger-soft); color: var(--color-danger-strong); }
.sign--pos { background: var(--color-success-soft); color: var(--color-success-strong); }
.sign--zero { background: var(--color-accent-soft); color: var(--color-accent-strong); }

.trend { font-size: 1.6rem; font-weight: 700; line-height: 1; }
.trend--dec { color: var(--color-danger); }
.trend--inc { color: var(--color-success); }

.sign-table[data-highlight="neg"] [data-col="neg"] { background: var(--color-danger-soft); }
.sign-table[data-highlight="pos"] [data-col="pos"] { background: var(--color-success-soft); }

.interval-picker__label { font-size: var(--text-sm); font-weight: 600; }
.stage__text .interval-picker__label { color: var(--color-text); }

.interval-picker { display: grid; gap: var(--space-3); }

.interval-btn {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  width: 100%;
  min-height: 56px;
  padding: var(--space-3) var(--space-5);
  text-align: left;
  font-weight: 600;
  background: var(--color-surface);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  cursor: pointer;
  transition: border-color var(--dur-fast), background var(--dur-fast);
}
.interval-btn .math { font-size: 1.2rem; min-width: 5.2em; }
.interval-btn__arrow { color: var(--color-text-muted); }
.interval-btn:hover { border-color: var(--color-accent); }
.interval-btn--dec[aria-pressed="true"] { border-color: var(--color-danger); background: var(--color-danger-soft); color: var(--color-danger-strong); }
.interval-btn--inc[aria-pressed="true"] { border-color: var(--color-success); background: var(--color-success-soft); color: var(--color-success-strong); }
.interval-btn.is-seen::after {
  content: "✓";
  margin-left: auto;
  font-size: 0.85rem;
  color: var(--color-text-muted);
}

/* ---------- Final challenge (stage 6) ---------- */

.signline {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  margin: var(--space-2) 0 var(--space-6);
  font-family: var(--font-math);
  color: var(--color-text-muted);
}
.signline__end { font-size: var(--text-sm); flex-shrink: 0; }
.signline__track {
  position: relative;
  flex: 1;
  height: 3px;
  background: var(--color-border-strong);
  border-radius: 2px;
}
.signline__point {
  position: absolute;
  left: var(--pos);
  top: 50%;
  width: 14px; height: 14px;
  transform: translate(-50%, -50%);
  border-radius: 50%;
  background: var(--color-surface);
  border: 3px solid var(--color-accent);
}
.signline__point span {
  position: absolute;
  top: 16px;
  left: 50%;
  transform: translateX(-50%);
  font-size: 1.05rem;
  font-weight: 600;
  color: var(--color-text);
}

.intervals {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: var(--space-3);
  margin-bottom: var(--space-4);
}

.interval-card {
  min-width: 0;
  margin: 0;
  padding: var(--space-3);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  background: var(--color-surface);
  display: flex;
  flex-direction: column;
  gap: var(--space-2);
  transition: border-color var(--dur-med), background var(--dur-med);
}
.interval-card__label {
  padding: 0;
  font-size: 1.1rem;
  font-weight: 600;
  text-align: center;
  width: 100%;
  float: left; /* keeps legend inside the fieldset box */
}
.interval-card__sign {
  clear: both;
  text-align: center;
  font-size: var(--text-sm);
  color: var(--color-text-muted);
}
.interval-card__sign-value {
  display: inline-grid;
  place-items: center;
  min-width: 26px; height: 26px;
  margin-left: var(--space-1);
  border-radius: 50%;
  background: var(--color-surface-muted);
  border: 1px dashed var(--color-border-strong);
  font-weight: 700;
  color: var(--color-text-muted);
}
.interval-card__sign-value.is-revealed { border-style: solid; animation: pop-in var(--dur-med) var(--ease); }
.interval-card__sign-value.is-revealed[data-sign="+"] { background: var(--color-success-soft); border-color: transparent; color: var(--color-success-strong); }
.interval-card__sign-value.is-revealed[data-sign="−"] { background: var(--color-danger-soft); border-color: transparent; color: var(--color-danger-strong); }

.choice-group { display: grid; gap: var(--space-2); }

.choice {
  min-height: 44px;
  padding: var(--space-2);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-sm);
  background: var(--color-surface-muted);
  font-size: var(--text-sm);
  font-weight: 600;
  cursor: pointer;
  transition: border-color var(--dur-fast), background var(--dur-fast), color var(--dur-fast);
}
.choice:hover:not(:disabled) { border-color: var(--color-accent); }
.choice:disabled { cursor: default; }
.choice--inc[aria-pressed="true"] { background: var(--color-success-soft); border-color: var(--color-success); color: var(--color-success-strong); }
.choice--dec[aria-pressed="true"] { background: var(--color-danger-soft); border-color: var(--color-danger); color: var(--color-danger-strong); }
.choice-group.is-locked .choice[aria-pressed="false"] { opacity: 0.45; }

.interval-card.is-wrong {
  border-color: var(--color-danger);
  background: var(--color-danger-soft);
  animation: nudge 360ms var(--ease);
}
.interval-card.is-correct {
  border-color: var(--color-success);
  background: var(--color-success-soft);
}

.interval-card__hint:empty { display: none; }
.interval-card__hint {
  font-size: var(--text-xs);
  line-height: 1.45;
  color: var(--color-danger-strong);
  text-align: center;
}

/* ---------- Summary ---------- */

.summary { padding: var(--space-7) var(--space-6); text-align: left; }
.summary .stage__title { margin-bottom: var(--space-5); }
.summary__badge {
  display: grid;
  place-items: center;
  width: 56px; height: 56px;
  margin-bottom: var(--space-4);
  border-radius: 50%;
  background: var(--color-success-soft);
  color: var(--color-success);
  animation: pop-in var(--dur-slow) var(--ease);
}
.summary__subtitle {
  margin: var(--space-6) 0 var(--space-3);
  font-weight: 650;
}
.steps {
  list-style: none;
  counter-reset: step;
  margin: 0 0 var(--space-6);
  padding: 0;
  display: grid;
  gap: var(--space-2);
}
.steps li {
  counter-increment: step;
  display: flex;
  align-items: center;
  gap: var(--space-4);
  padding: var(--space-3) var(--space-4);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  background: var(--color-surface-muted);
}
.steps li::before {
  content: counter(step);
  display: grid;
  place-items: center;
  width: 30px; height: 30px;
  flex-shrink: 0;
  border-radius: 50%;
  background: var(--color-accent);
  color: #fff;
  font-weight: 700;
  font-size: var(--text-sm);
}

/* ---------- Buttons ---------- */

.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2);
  min-height: 50px;
  padding: 0 var(--space-6);
  border: 0;
  border-radius: var(--radius-md);
  font-weight: 650;
  cursor: pointer;
  transition: background var(--dur-fast), transform var(--dur-fast), opacity var(--dur-fast), box-shadow var(--dur-fast);
}
.btn:active:not(:disabled) { transform: translateY(1px); }

.btn--primary {
  background: var(--color-accent);
  color: #fff;
  box-shadow: 0 4px 14px rgba(80, 70, 214, 0.28);
}
.btn--primary:hover:not(:disabled) { background: var(--color-accent-strong); }
.btn--primary:disabled {
  background: #c9cde0;
  box-shadow: none;
  cursor: not-allowed;
}

.btn--secondary {
  background: var(--color-text);
  color: #fff;
}
.btn--secondary:hover:not(:disabled) { background: #26325a; }
.btn--secondary:disabled { opacity: 0.4; cursor: default; }

.btn--block { width: 100%; }

/* ---------- Action bar ---------- */

.actionbar {
  position: fixed;
  left: 0; right: 0; bottom: 0;
  z-index: 20;
  background: rgba(255, 255, 255, 0.94);
  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);
  border-top: 1px solid var(--color-border);
  padding-bottom: env(safe-area-inset-bottom);
  transition: background var(--dur-med);
}
.actionbar[hidden] { display: none; }
.actionbar.is-complete { background: rgba(231, 246, 239, 0.96); }

.actionbar__inner {
  max-width: 1180px;
  min-height: var(--actionbar-height);
  margin: 0 auto;
  padding: var(--space-3) var(--space-5);
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-4);
}
.actionbar__hint {
  font-size: var(--text-sm);
  color: var(--color-text-muted);
  display: flex;
  align-items: center;
  gap: var(--space-2);
}
.actionbar.is-complete .actionbar__hint { color: var(--color-success-strong); font-weight: 600; }
.actionbar__inner .btn { min-width: 180px; flex-shrink: 0; }

/* ---------- Keyframes ---------- */

@keyframes fade-in {
  from { opacity: 0; transform: translateY(6px); }
  to   { opacity: 1; transform: none; }
}
@keyframes pop-in {
  from { transform: scale(0.94); }
  to   { transform: scale(1); }
}
@keyframes nudge {
  0%, 100% { transform: translateX(0); }
  30% { transform: translateX(-4px); }
  60% { transform: translateX(3px); }
}

/* ---------- Responsive ---------- */

@media (max-width: 960px) {
  .stage.is-active {
    grid-template-columns: minmax(0, 1fr);
    gap: var(--space-5);
  }
  .stage__text { padding-top: 0; }
  .lesson { padding-top: var(--space-5); }
}

@media (max-width: 640px) {
  :root { --actionbar-height: 92px; }
  body { font-size: 1rem; }

  .topbar__inner { flex-wrap: wrap; gap: var(--space-2) var(--space-4); padding: var(--space-3) var(--space-4); }
  .brand__mark { width: 32px; height: 32px; font-size: 1rem; }
  .progress { flex-basis: 100%; }

  .lesson { padding-left: var(--space-4); padding-right: var(--space-4); }
  .card { padding: var(--space-4); border-radius: var(--radius-md); }
  .rule-card { padding: var(--space-4); }
  .rule {
    grid-template-columns: 1fr;
    gap: var(--space-1);
    padding: var(--space-3) var(--space-4);
  }
  .rule__arrow { display: none; }

  .readout__cell { padding: var(--space-2) var(--space-3); }
  .readout__cell dd { font-size: 1.25rem; }

  .intervals { grid-template-columns: minmax(0, 1fr); }
  .interval-card { flex-direction: row; flex-wrap: wrap; align-items: center; }
  .interval-card__label { width: auto; text-align: left; }
  .interval-card__sign { margin-left: auto; }
  .choice-group { grid-template-columns: 1fr 1fr; flex-basis: 100%; }
  .interval-card__hint { flex-basis: 100%; }

  .summary { padding: var(--space-5) var(--space-4); }

  .actionbar__inner { flex-direction: column; align-items: stretch; gap: var(--space-1); padding: var(--space-2) var(--space-4); }
  .actionbar__hint { justify-content: center; text-align: center; font-size: var(--text-xs); }
  .actionbar__hint:empty { display: none; }
  .actionbar__inner .btn { width: 100%; min-width: 0; }
}

/* =========================================================
   Shared components added for lesson 2.1 (reusable in later lessons)
   ========================================================= */

/* ---------- Split stage: text + task on the left, visual on the right.
   On mobile the order becomes text → visual → task. ---------- */

.stage--split.is-active {
  grid-template-areas:
    "text visual"
    "task visual";
  grid-template-rows: auto 1fr;
  row-gap: var(--space-5);
}
.stage--split .stage__text { grid-area: text; }
.stage--split .stage__activity { grid-area: visual; }
.stage--split .stage__task { grid-area: task; }
.stage__task > * + * { margin-top: var(--space-4); }

@media (max-width: 960px) {
  .stage--split.is-active {
    grid-template-areas: "text" "visual" "task";
    grid-template-rows: auto;
  }
}

@media (min-width: 961px) {
  .stage--wide.is-active {
    grid-template-columns: minmax(0, 0.78fr) minmax(0, 1.22fr);
    column-gap: var(--space-6);
  }
}

.stage__activity > * + * { margin-top: var(--space-4); }

/* ---------- Math: fractions, limits, formula cards ---------- */

.frac {
  display: inline-flex;
  flex-direction: column;
  align-items: stretch;
  vertical-align: middle;
  margin: 0 0.12em;
  text-align: center;
  line-height: 1.25;
}
.frac > span:first-child {
  padding: 0 0.3em 0.08em;
  border-bottom: 1.5px solid currentColor;
}
.frac > span:last-child { padding: 0.1em 0.3em 0; }

.lim {
  display: inline-flex;
  flex-direction: column;
  align-items: center;
  vertical-align: middle;
  margin-right: 0.18em;
  line-height: 1.05;
  font-style: normal;
}
.lim > span:last-child {
  font-size: 0.62em;
  white-space: nowrap;
}

.formula {
  font-family: var(--font-math);
  font-size: 1.3rem;
  line-height: 2.1;
  color: var(--color-text);
}
.formula .math { font-size: 1em; }

.formula-card {
  padding: var(--space-5);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-md);
}
.formula-card--accent {
  background: linear-gradient(180deg, var(--color-accent-soft), var(--color-surface) 75%);
  border-color: rgba(80, 70, 214, 0.25);
}
.formula-card__label {
  font-size: var(--text-xs);
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--color-accent);
  margin-bottom: var(--space-3);
}
.formula-card__note {
  margin-top: var(--space-3);
  font-size: var(--text-sm);
  color: var(--color-text-muted);
}
.formula--hero { font-size: clamp(1.25rem, 1rem + 1vw, 1.6rem); text-align: center; }

.is-revealing { animation: fade-in var(--dur-slow) var(--ease); }

/* ---------- Segmented toggle ---------- */

.segmented {
  display: inline-grid;
  grid-auto-flow: column;
  grid-auto-columns: 1fr;
  gap: var(--space-1);
  padding: var(--space-1);
  background: var(--color-surface-muted);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
}
.segmented button {
  min-height: 42px;
  padding: 0 var(--space-4);
  border: 0;
  border-radius: var(--radius-sm);
  background: transparent;
  font-weight: 600;
  font-size: var(--text-sm);
  color: var(--color-text-muted);
  cursor: pointer;
  transition: background var(--dur-fast), color var(--dur-fast), box-shadow var(--dur-fast);
}
.segmented button[aria-pressed="true"] {
  background: var(--color-surface);
  color: var(--color-accent-strong);
  box-shadow: var(--shadow-sm);
}

.chip[aria-pressed="true"] {
  background: var(--color-accent);
  border-color: var(--color-accent);
  color: #fff;
}
.chip.is-visited:not([aria-pressed="true"]) { border-color: var(--color-accent); }

/* ---------- Readout variants ---------- */

.readout--4 { grid-template-columns: repeat(4, minmax(0, 1fr)); }
.readout--2 { grid-template-columns: repeat(2, minmax(0, 1fr)); }
.readout__cell--accent { background: var(--color-accent-soft); border-color: rgba(80, 70, 214, 0.3); }
.readout__cell--accent dd { color: var(--color-accent-strong); }
.readout__cell dd.is-ticking { animation: tick var(--dur-med) var(--ease); }

@keyframes tick {
  from { opacity: 0.35; transform: translateY(-3px); }
  to   { opacity: 1; transform: none; }
}

/* A calculation line such as ΔS = … = … */
.calc {
  display: grid;
  gap: var(--space-2);
  padding: var(--space-4);
  border-radius: var(--radius-md);
  background: var(--color-surface-muted);
  border: 1px solid var(--color-border);
}
.calc .formula { font-size: 1.12rem; }
/* A single calculation line (inline text + math) must flow as text, not as grid cells. */
p.calc { display: block; line-height: 1.9; overflow-wrap: anywhere; }
.calc__result {
  padding: 0.05em 0.4em;
  border-radius: var(--radius-sm);
  background: var(--color-accent-soft);
  color: var(--color-accent-strong);
  font-weight: 600;
}

/* ---------- Motion controls ---------- */

.controls-row {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: var(--space-3);
}

.btn--ghost {
  min-height: 42px;
  padding: 0 var(--space-4);
  background: var(--color-surface);
  border: 1px solid var(--color-border-strong);
  color: var(--color-text);
  font-size: var(--text-sm);
}
.btn--ghost:hover:not(:disabled) { border-color: var(--color-accent); background: var(--color-accent-soft); }
.btn--ghost:disabled { opacity: 0.5; cursor: default; }

.graph.graph--track, .graph-card .graph.graph--track { aspect-ratio: 16 / 6; }
.graph.graph--track-tall, .graph-card .graph.graph--track-tall { aspect-ratio: 16 / 8; }
@media (max-width: 640px) {
  .graph.graph--track, .graph-card .graph.graph--track { aspect-ratio: 16 / 8; }
  .graph.graph--track-tall, .graph-card .graph.graph--track-tall { aspect-ratio: 16 / 11; }
}

/* ---------- Data table (values converging) ---------- */

.data-table {
  width: 100%;
  border-collapse: collapse;
  font-variant-numeric: tabular-nums;
  text-align: center;
}
.data-table th {
  padding: var(--space-2) var(--space-2) var(--space-3);
  font-size: var(--text-sm);
  font-weight: 600;
  color: var(--color-text-muted);
  border-bottom: 1px solid var(--color-border);
}
.data-table td {
  padding: var(--space-2);
  font-family: var(--font-math);
  font-size: 1.1rem;
  border-bottom: 1px solid var(--color-border);
}
.data-table tr:last-child td { border-bottom: 0; }
.data-table tbody tr { animation: fade-in var(--dur-med) var(--ease); }
.data-table tbody tr.is-latest td { background: var(--color-accent-soft); }
.data-table tbody tr.is-latest td:last-child { color: var(--color-accent-strong); font-weight: 600; }
.data-table__empty td { color: var(--color-text-muted); font-family: var(--font-sans); font-size: var(--text-sm); }

/* ---------- Quiz lock (prerequisite not done yet) ---------- */

.quiz.is-locked .options { opacity: 0.5; }
.quiz__lock-note {
  display: none;
  margin: calc(-1 * var(--space-3)) 0 var(--space-4);
  font-size: var(--text-sm);
  color: var(--color-text-muted);
}
.quiz.is-locked .quiz__lock-note { display: block; }

.options--inline { grid-template-columns: repeat(auto-fit, minmax(96px, 1fr)); }

/* ---------- Worked derivation (step by step) ---------- */

.derivation {
  display: grid;
  gap: var(--space-3);
  margin: 0;
  padding: 0;
  list-style: none;
}
.derivation__line {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  padding: var(--space-3) var(--space-4);
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
  background: var(--color-surface-muted);
  overflow-x: auto;
}
.derivation__line .formula { font-size: 1.15rem; white-space: nowrap; }
.derivation__line.is-new {
  animation: fade-in var(--dur-slow) var(--ease);
  border-color: rgba(80, 70, 214, 0.35);
  background: var(--color-accent-soft);
}
.derivation__line.is-final {
  border-color: rgba(30, 148, 103, 0.4);
  background: var(--color-success-soft);
}
.derivation__line.is-final .formula { color: var(--color-success-strong); font-weight: 600; }
.derivation__check {
  display: grid;
  place-items: center;
  width: 24px; height: 24px;
  flex-shrink: 0;
  border-radius: 50%;
  background: var(--color-success);
  color: #fff;
  font-size: 0.8rem;
  font-weight: 700;
}
.derivation__line.is-pending { border-style: dashed; background: transparent; }
.derivation__line.is-pending .formula { color: var(--color-text-muted); }

.cancel {
  position: relative;
  display: inline-block;
}
.cancel::after {
  content: "";
  position: absolute;
  left: -2px; right: -2px;
  top: 50%;
  height: 2px;
  background: var(--color-danger);
  transform: rotate(-18deg) scaleX(0);
  transform-origin: left center;
  animation: strike var(--dur-slow) var(--ease) 250ms forwards;
}
@keyframes strike { to { transform: rotate(-18deg) scaleX(1); } }

.step-counter {
  font-size: var(--text-xs);
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--color-text-muted);
  margin-bottom: var(--space-2);
}

/* ---------- Logic diagram (⇒ / ⇏) ---------- */

.logic {
  display: grid;
  grid-template-columns: minmax(0, 1fr) auto minmax(0, 1fr);
  align-items: center;
  gap: var(--space-3);
}
.logic__box {
  padding: var(--space-4);
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
  background: var(--color-surface-muted);
  text-align: center;
  font-weight: 650;
  line-height: 1.35;
}
.logic__arrows { display: grid; gap: var(--space-3); justify-items: center; }
.logic__arrow {
  display: grid;
  justify-items: center;
  gap: 2px;
  min-width: 96px;
  padding: var(--space-2) var(--space-3);
  border-radius: var(--radius-sm);
  border: 1px dashed var(--color-border-strong);
  color: var(--color-text-muted);
  font-size: var(--text-xs);
  font-weight: 600;
  transition: border-color var(--dur-med), background var(--dur-med), color var(--dur-med);
}
.logic__arrow-glyph { font-size: 1.5rem; line-height: 1; font-family: var(--font-math); }
.logic__arrow.is-true {
  border-style: solid;
  border-color: var(--color-success);
  background: var(--color-success-soft);
  color: var(--color-success-strong);
  animation: pop-in var(--dur-med) var(--ease);
}
.logic__arrow.is-false {
  border-style: solid;
  border-color: var(--color-danger);
  background: var(--color-danger-soft);
  color: var(--color-danger-strong);
  animation: pop-in var(--dur-med) var(--ease);
}
.logic__caption {
  margin-top: var(--space-4);
  font-size: var(--text-sm);
  color: var(--color-text-muted);
  text-align: center;
}

@media (max-width: 520px) {
  .logic { grid-template-columns: minmax(0, 1fr); }
  .logic__arrows { grid-auto-flow: column; }
}

.proof {
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  background: var(--color-surface);
}
.proof summary {
  padding: var(--space-3) var(--space-4);
  font-weight: 600;
  font-size: var(--text-sm);
  cursor: pointer;
  color: var(--color-accent-strong);
}
.proof__body {
  padding: 0 var(--space-4) var(--space-4);
  font-size: var(--text-sm);
  color: var(--color-text-muted);
}
.proof__body .formula { font-size: 1.05rem; margin: var(--space-2) 0; }

/* ---------- Exam tip ---------- */

.exam-tip {
  position: relative;
  padding: var(--space-4) var(--space-5) var(--space-4) calc(var(--space-5) + 4px);
  border-radius: var(--radius-md);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  font-size: var(--text-sm);
}
.exam-tip::before {
  content: "";
  position: absolute;
  left: 0; top: 12px; bottom: 12px;
  width: 4px;
  border-radius: 0 4px 4px 0;
  background: #b7800f;
}
.exam-tip__label {
  font-size: var(--text-xs);
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: #8a5f06;
  margin-bottom: var(--space-1);
}
.stage__text .exam-tip p:not(.exam-tip__label) { color: var(--color-text); }

/* ---------- Key statement (e.g. “continuous but not differentiable”) ---------- */

.keypoint {
  display: flex;
  gap: var(--space-3);
  align-items: flex-start;
  padding: var(--space-4);
  border-radius: var(--radius-md);
  background: var(--color-accent-soft);
  border: 1px solid rgba(80, 70, 214, 0.22);
  font-size: var(--text-sm);
}
.keypoint strong { color: var(--color-accent-strong); }

/* ---------- Final challenge (submit once, then score) ---------- */

.challenge { display: grid; gap: var(--space-4); }
.challenge-q {
  margin: 0;
  padding: var(--space-4) var(--space-5);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  background: var(--color-surface);
  transition: border-color var(--dur-med), background var(--dur-med);
}
.challenge-q legend {
  float: left;
  width: 100%;
  padding: 0;
  margin-bottom: var(--space-3);
  font-weight: 650;
  line-height: 1.45;
}
.challenge-q__num {
  display: inline-grid;
  place-items: center;
  width: 26px; height: 26px;
  margin-right: var(--space-2);
  border-radius: 50%;
  background: var(--color-accent-soft);
  color: var(--color-accent-strong);
  font-size: var(--text-xs);
  font-weight: 700;
}
.challenge-q .options { clear: both; gap: var(--space-2); }
.challenge-q .option { min-height: 50px; }
.option[aria-pressed="true"] {
  border-color: var(--color-accent);
  background: var(--color-accent-soft);
}
.option[aria-pressed="true"] .option__key { background: var(--color-accent); border-color: var(--color-accent); color: #fff; }
.option.is-correct[aria-pressed="true"] { border-color: var(--color-success); background: var(--color-success-soft); }
.option.is-correct[aria-pressed="true"] .option__key { background: var(--color-success); border-color: var(--color-success); }
.option.is-wrong[aria-pressed="true"] { border-color: var(--color-danger); background: var(--color-danger-soft); }
.option.is-wrong[aria-pressed="true"] .option__key { background: var(--color-danger); border-color: var(--color-danger); }
.option.is-answer { border-color: var(--color-success); border-style: dashed; }
.option.is-answer .option__status::before { content: "✓"; color: var(--color-success-strong); }
.challenge-q.is-correct { border-color: var(--color-success); background: var(--color-success-soft); }
.challenge-q.is-wrong { border-color: var(--color-danger); background: var(--color-danger-soft); }
.challenge-q__verdict:empty { display: none; }
.challenge-q__verdict {
  margin-top: var(--space-3);
  font-size: var(--text-sm);
  line-height: 1.5;
}
.challenge-q__verdict strong { margin-right: var(--space-1); }
.challenge-q.is-correct .challenge-q__verdict strong { color: var(--color-success-strong); }
.challenge-q.is-wrong .challenge-q__verdict strong { color: var(--color-danger-strong); }

.score {
  display: flex;
  align-items: center;
  gap: var(--space-4);
  padding: var(--space-4) var(--space-5);
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
  background: var(--color-surface-muted);
  animation: fade-in var(--dur-slow) var(--ease);
}
.score__value {
  white-space: nowrap;
  font-size: 2rem;
  font-weight: 700;
  font-variant-numeric: tabular-nums;
  color: var(--color-accent-strong);
}
.score__text { font-size: var(--text-sm); color: var(--color-text-muted); }
.score__text strong { color: var(--color-text); display: block; font-size: var(--text-md); }

/* ---------- Summary flow (connected cards) ---------- */

.flow { display: grid; gap: 0; margin-bottom: var(--space-5); }
.flow__card {
  display: flex;
  align-items: center;
  gap: var(--space-4);
  padding: var(--space-4) var(--space-5);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  background: var(--color-surface-muted);
}
.flow__card--final { background: var(--color-accent-soft); border-color: rgba(80, 70, 214, 0.3); }
.flow__num {
  display: grid;
  place-items: center;
  width: 30px; height: 30px;
  flex-shrink: 0;
  border-radius: 50%;
  background: var(--color-accent);
  color: #fff;
  font-weight: 700;
  font-size: var(--text-sm);
}
.flow__title { font-weight: 650; }
.flow__card .formula { font-size: 1.1rem; margin-top: var(--space-1); }
.flow__arrow {
  justify-self: center;
  padding: var(--space-1) 0;
  color: var(--color-text-muted);
  font-size: 1.2rem;
  line-height: 1;
}

.meanings {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: var(--space-3);
  margin-bottom: var(--space-5);
}
.meaning {
  padding: var(--space-4);
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
}
.meaning__label {
  font-size: var(--text-xs);
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--color-text-muted);
  margin-bottom: var(--space-1);
}
.meaning .formula { font-size: 1.1rem; margin-top: var(--space-1); }

.remember { display: grid; gap: var(--space-2); margin-bottom: var(--space-6); }
.remember__item {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  padding: var(--space-3) var(--space-4);
  border-radius: var(--radius-md);
  font-weight: 600;
}
.remember__item--yes { background: var(--color-success-soft); color: var(--color-success-strong); }
.remember__item--no { background: var(--color-danger-soft); color: var(--color-danger-strong); }
.remember__icon {
  display: grid;
  place-items: center;
  width: 26px; height: 26px;
  flex-shrink: 0;
  border-radius: 50%;
  color: #fff;
  font-size: 0.85rem;
}
.remember__item--yes .remember__icon { background: var(--color-success); }
.remember__item--no .remember__icon { background: var(--color-danger); }

@media (max-width: 640px) {
  .readout--4 { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .meanings { grid-template-columns: minmax(0, 1fr); }
  .formula { font-size: 1.15rem; }
  .challenge-q { padding: var(--space-4); }
  .flow__card { padding: var(--space-3) var(--space-4); }
}

.stage--single.is-active {
  grid-template-columns: minmax(0, 760px);
  justify-content: center;
}
.formula i { font-style: italic; padding-right: 0.09em; }
.readout__cell--alt { background: #fbf1e6; border-color: rgba(194, 122, 44, 0.35); }
.readout__cell--alt dd { color: #8f5519; }

/* =========================================================
   Components added for 1.1 Πραγματικοί αριθμοί (reusable: sets, number lines, intervals)
   ========================================================= */

/* ---------- Classify a number (one card at a time) ---------- */

.classify__head {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: var(--space-3);
  margin-bottom: var(--space-4);
}
.progress-dots { display: flex; gap: 6px; margin: 0; padding: 0; list-style: none; }
.progress-dots li {
  width: 10px; height: 10px;
  border-radius: 50%;
  background: var(--color-border);
  transition: background var(--dur-med);
}
.progress-dots li.is-current { background: var(--color-accent); }
.progress-dots li.is-done { background: var(--color-success); }

.big-number {
  display: grid;
  place-items: center;
  min-height: 110px;
  margin-bottom: var(--space-4);
  border-radius: var(--radius-md);
  background: var(--color-surface-muted);
  border: 1px solid var(--color-border);
  font-family: var(--font-math);
  font-size: clamp(2.4rem, 1.8rem + 2vw, 3.2rem);
  color: var(--color-text);
}
.big-number.is-revealing { animation: pop-in var(--dur-med) var(--ease); }

.set-options { grid-template-columns: repeat(2, minmax(0, 1fr)); }
.set-options .option__text small { display: block; font-size: var(--text-xs); color: var(--color-text-muted); font-weight: 500; }

.membership {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: var(--space-2);
  margin: var(--space-4) 0 0;
  padding: 0;
  list-style: none;
}
.membership li {
  display: grid;
  justify-items: center;
  gap: 2px;
  padding: var(--space-2);
  border-radius: var(--radius-sm);
  border: 1px dashed var(--color-border-strong);
  font-family: var(--font-math);
  font-size: 1.15rem;
  color: var(--color-text-muted);
  transition: background var(--dur-med), border-color var(--dur-med), color var(--dur-med);
}
.membership li small { font-family: var(--font-sans); font-size: var(--text-xs); font-weight: 600; }
.membership li.is-member { border-style: solid; border-color: rgba(30, 148, 103, 0.4); background: var(--color-success-soft); color: var(--color-success-strong); }
.membership li.is-not-member { border-style: solid; background: var(--color-surface-muted); text-decoration: line-through; }

.next-row { display: flex; justify-content: flex-end; margin-top: var(--space-4); }

/* ---------- Nested number sets ---------- */

.nest__set {
  position: relative;
  padding: var(--space-3) var(--space-3) var(--space-3);
  border-radius: var(--radius-lg);
  border: 1.5px solid var(--color-border-strong);
  cursor: pointer;
  transition: background var(--dur-med), border-color var(--dur-med), box-shadow var(--dur-med);
}
.nest__set .nest__set { margin-top: var(--space-2); }
.nest__set--R { background: #f7f8fc; }
.nest__set--Q { background: #f0f2fb; }
.nest__set--Z { background: #e9ecfa; }
.nest__set--N { background: #e2e6f9; }
.nest__set.is-selected {
  border-color: var(--color-accent);
  box-shadow: 0 0 0 3px rgba(80, 70, 214, 0.18);
}
.nest__set.is-selected .nest__set { border-color: rgba(80, 70, 214, 0.55); }
.nest__head { display: flex; align-items: center; gap: var(--space-2); flex-wrap: wrap; }
.nest__tab {
  display: inline-flex;
  align-items: center;
  gap: var(--space-2);
  min-height: 38px;
  padding: 0 var(--space-3);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-pill);
  background: var(--color-surface);
  font-weight: 650;
  font-size: var(--text-sm);
  cursor: pointer;
}
.nest__tab .math { font-size: 1.2rem; }
.nest__tab[aria-pressed="true"] { background: var(--color-accent); border-color: var(--color-accent); color: #fff; }
.nest__caption { font-size: var(--text-xs); color: var(--color-text-muted); }
.nest__items { display: flex; flex-wrap: wrap; gap: 6px; margin-top: var(--space-2); }

.num-chip {
  display: inline-grid;
  place-items: center;
  min-width: 38px;
  min-height: 30px;
  padding: 0 8px;
  border-radius: var(--radius-sm);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  font-family: var(--font-math);
  font-size: 1rem;
  transition: opacity var(--dur-med);
}
.num-chip--irrational { border-style: dashed; border-color: var(--color-accent); color: var(--color-accent-strong); }
.num-chip.is-removed { opacity: 0.35; text-decoration: line-through; }

.set-chain {
  font-family: var(--font-math);
  font-size: clamp(1.4rem, 1.1rem + 1.2vw, 1.9rem);
  color: var(--color-text);
  letter-spacing: 0.02em;
}
.set-chain span { padding: 0 0.1em; border-radius: 6px; transition: background var(--dur-med), color var(--dur-med); }
.set-chain span.is-on { background: var(--color-accent-soft); color: var(--color-accent-strong); }
.set-info__title { font-weight: 700; margin-bottom: var(--space-2); }
.set-info__def { font-family: var(--font-math); font-size: 1.15rem; margin-bottom: var(--space-2); }

.switch-btn {
  display: inline-flex;
  align-items: center;
  gap: var(--space-3);
  min-height: 44px;
  padding: 0 var(--space-4);
  border: 1px solid var(--color-border-strong);
  border-radius: var(--radius-pill);
  background: var(--color-surface);
  font-weight: 600;
  font-size: var(--text-sm);
  cursor: pointer;
}
.switch-btn__track {
  position: relative;
  width: 36px; height: 20px;
  border-radius: var(--radius-pill);
  background: var(--color-border-strong);
  transition: background var(--dur-fast);
}
.switch-btn__track::after {
  content: "";
  position: absolute;
  top: 3px; left: 3px;
  width: 14px; height: 14px;
  border-radius: 50%;
  background: #fff;
  transition: transform var(--dur-fast) var(--ease);
}
.switch-btn[aria-pressed="true"] .switch-btn__track { background: var(--color-accent); }
.switch-btn[aria-pressed="true"] .switch-btn__track::after { transform: translateX(16px); }

/* ---------- Sorting rows (ρητός / άρρητος) ---------- */

.sort-list { display: grid; gap: var(--space-2); margin: 0; padding: 0; list-style: none; }
.sort-row {
  display: grid;
  grid-template-columns: 5.5rem minmax(0, 1fr);
  align-items: center;
  gap: var(--space-2) var(--space-4);
  padding: var(--space-3) var(--space-4);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  background: var(--color-surface);
  transition: border-color var(--dur-med), background var(--dur-med);
}
.sort-row__num { font-family: var(--font-math); font-size: 1.35rem; text-align: center; }
.sort-row__buttons { display: grid; grid-template-columns: 1fr 1fr; gap: var(--space-2); }
.sort-row__note { grid-column: 1 / -1; font-size: var(--text-sm); color: var(--color-text-muted); }
.sort-row__note:empty { display: none; }
.sort-row.is-correct { border-color: var(--color-success); background: var(--color-success-soft); }
.sort-row.is-correct .sort-row__note { color: var(--color-success-strong); }
.sort-row.is-wrong .sort-row__note { color: var(--color-danger-strong); }

.choice.is-correct { background: var(--color-success); border-color: var(--color-success); color: #fff; }
.choice.is-correct::before { content: "✓ "; }
.choice.is-wrong { background: var(--color-danger-soft); border-color: var(--color-danger); color: var(--color-danger-strong); }
.choice.is-wrong::before { content: "✕ "; }

/* ---------- Order in ℝ ---------- */

.op-buttons { display: flex; flex-wrap: wrap; gap: var(--space-2); }
.op-buttons .chip { font-family: var(--font-math); min-width: 64px; }
.ineq-display {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-3);
  padding: var(--space-4);
  border-radius: var(--radius-md);
  background: var(--color-surface-muted);
  border: 1px solid var(--color-border);
  font-family: var(--font-math);
  font-size: clamp(1.6rem, 1.3rem + 1.2vw, 2.2rem);
}
.ineq-display__a { color: var(--color-accent-strong); }
.ineq-display__b { color: #8f5519; }
.ineq-display__rel { color: var(--color-text); font-weight: 600; }
.ineq-display__rel.is-flipped { color: var(--color-danger-strong); animation: pop-in var(--dur-med) var(--ease); }
.step-log { margin: 0; padding: 0; list-style: none; display: grid; gap: var(--space-1); font-size: var(--text-sm); color: var(--color-text-muted); }
.step-log li { font-family: var(--font-math); }
.step-log li.is-flip { color: var(--color-danger-strong); }

.props-list { margin: 0; padding-left: var(--space-5); display: grid; gap: var(--space-2); }
.props-list li { font-size: var(--text-sm); color: var(--color-text-muted); }
.props-list .math { color: var(--color-text); }

/* ---------- Interval notation display ---------- */

.notation {
  display: grid;
  gap: var(--space-2);
  padding: var(--space-4);
  border-radius: var(--radius-md);
  background: var(--color-accent-soft);
  border: 1px solid rgba(80, 70, 214, 0.25);
  text-align: center;
}
.notation__ineq { font-family: var(--font-math); font-size: 1.25rem; color: var(--color-text); }
.notation__interval { font-family: var(--font-math); font-size: clamp(1.8rem, 1.4rem + 1.4vw, 2.4rem); font-weight: 600; color: var(--color-accent-strong); }
.notation__interval.is-revealing { animation: pop-in var(--dur-med) var(--ease); }
.notation__set { font-family: var(--font-math); font-size: 1rem; color: var(--color-text-muted); }
.notation__name { font-size: var(--text-sm); font-weight: 650; color: var(--color-accent-strong); }

.endpoint-toggles { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: var(--space-3); }
.endpoint-toggle {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  min-height: 52px;
  padding: var(--space-2) var(--space-3);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  background: var(--color-surface);
  text-align: left;
  font-size: var(--text-sm);
  font-weight: 600;
  cursor: pointer;
  transition: border-color var(--dur-fast), background var(--dur-fast);
}
.endpoint-toggle:hover { border-color: var(--color-accent); }
.endpoint-toggle__dot {
  width: 18px; height: 18px;
  flex-shrink: 0;
  border-radius: 50%;
  border: 3px solid var(--color-accent);
  background: #fff;
}
.endpoint-toggle[aria-pressed="true"] { border-color: var(--color-accent); background: var(--color-accent-soft); }
.endpoint-toggle[aria-pressed="true"] .endpoint-toggle__dot { background: var(--color-accent); }
.endpoint-toggle small { display: block; font-weight: 500; color: var(--color-text-muted); font-size: var(--text-xs); }

.ray-options { display: grid; grid-template-columns: repeat(auto-fit, minmax(92px, 1fr)); gap: var(--space-2); }
.ray-options .chip { font-family: var(--font-math); min-height: 46px; }

/* ---------- Piecewise definition ---------- */

.cases { display: inline-flex; align-items: center; gap: 0.25em; vertical-align: middle; font-family: var(--font-math); }
.cases__brace { font-size: 2.6em; line-height: 1; font-weight: 300; transform: scaleX(0.7); }
.cases__rows { display: grid; gap: 0.15em; text-align: left; }
.cases__rows span { white-space: nowrap; }

.live-check {
  padding: var(--space-3) var(--space-4);
  border-radius: var(--radius-md);
  background: var(--color-surface-muted);
  border: 1px solid var(--color-border);
  font-family: var(--font-math);
  font-size: 1.05rem;
  overflow-x: auto;
  white-space: nowrap;
}

.status-line {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  padding: var(--space-3) var(--space-4);
  border-radius: var(--radius-md);
  border: 2px solid var(--color-border);
  font-weight: 600;
  transition: background var(--dur-med), border-color var(--dur-med);
}
.status-line__icon {
  display: grid;
  place-items: center;
  width: 26px; height: 26px;
  flex-shrink: 0;
  border-radius: 50%;
  color: #fff;
  background: var(--color-border-strong);
  font-size: 0.85rem;
}
.status-line[data-state="in"] { border-color: var(--color-success); background: var(--color-success-soft); color: var(--color-success-strong); }
.status-line[data-state="in"] .status-line__icon { background: var(--color-success); }
.status-line[data-state="out"] { border-color: var(--color-danger); background: var(--color-danger-soft); color: var(--color-danger-strong); }
.status-line[data-state="out"] .status-line__icon { background: var(--color-danger); }

.chain-steps { display: grid; gap: var(--space-1); }
.chain-steps .formula { font-size: 1.15rem; line-height: 1.8; }

/* Very narrow phones: two set options per row only fit with tighter spacing. */
@media (max-width: 380px) {
  .set-options .option { gap: var(--space-2); padding-left: var(--space-3); padding-right: var(--space-3); }
}

@media (max-width: 640px) {
  .set-options { grid-template-columns: minmax(0, 1fr) minmax(0, 1fr); }
  .membership { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .sort-row { grid-template-columns: minmax(0, 1fr); }
  .sort-row__num { text-align: left; }
  .endpoint-toggles { grid-template-columns: minmax(0, 1fr); }
  .nest__set { padding: var(--space-2); }
}

/* =========================================================
   Components added for 1.2 Συναρτήσεις (reusable: function machine,
   mapping diagram, pipeline, gallery tabs)
   ========================================================= */

/* ---------- Function machine ---------- */

.machine {
  display: grid;
  grid-template-columns: minmax(0, 1fr) auto minmax(0, 1.2fr) auto minmax(0, 1fr);
  align-items: center;
  gap: var(--space-2);
}
.machine__io {
  display: grid;
  justify-items: center;
  gap: var(--space-1);
  padding: var(--space-3) var(--space-2);
  border-radius: var(--radius-md);
  border: 1.5px dashed var(--color-border-strong);
  background: var(--color-surface);
  min-height: 92px;
  align-content: center;
}
.machine__label { font-size: var(--text-xs); font-weight: 700; letter-spacing: 0.06em; text-transform: uppercase; color: var(--color-text-muted); }
.machine__value { font-family: var(--font-math); font-size: 1.6rem; color: var(--color-text); }
.machine__value.is-new { animation: pop-in var(--dur-med) var(--ease); color: var(--color-accent-strong); }
.machine__box {
  display: grid;
  justify-items: center;
  gap: var(--space-1);
  padding: var(--space-4) var(--space-3);
  border-radius: var(--radius-lg);
  background: linear-gradient(180deg, var(--color-accent), var(--color-accent-strong));
  color: #fff;
  box-shadow: 0 6px 18px rgba(80, 70, 214, 0.28);
  min-height: 104px;
  align-content: center;
}
.machine__name { font-family: var(--font-math); font-style: italic; font-size: 1.8rem; line-height: 1; }
.machine__rule { font-family: var(--font-math); font-size: 1.05rem; opacity: 0.92; white-space: nowrap; }
.machine__arrow { color: var(--color-text-muted); font-size: 1.4rem; }
.machine.is-running .machine__box { animation: machine-pulse 520ms var(--ease); }
.machine.is-running .machine__arrow { animation: arrow-flow 520ms var(--ease); }
.machine--blocked .machine__io:last-child { border-color: var(--color-danger); background: var(--color-danger-soft); }

@keyframes machine-pulse {
  40% { transform: scale(1.04); box-shadow: 0 8px 26px rgba(80, 70, 214, 0.42); }
}
@keyframes arrow-flow {
  0% { transform: translateX(-4px); opacity: 0.3; }
  100% { transform: none; opacity: 1; }
}

.pairs {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-2);
  margin: 0;
  padding: 0;
  list-style: none;
}
.pairs li {
  padding: 4px 10px;
  border-radius: var(--radius-pill);
  background: var(--color-accent-soft);
  color: var(--color-accent-strong);
  font-family: var(--font-math);
  animation: fade-in var(--dur-med) var(--ease);
}

.number-input { display: flex; align-items: center; gap: var(--space-2); }
.number-input input {
  width: 96px;
  min-height: 42px;
  padding: 0 var(--space-3);
  border: 1.5px solid var(--color-border-strong);
  border-radius: var(--radius-sm);
  font: inherit;
  font-family: var(--font-math);
  font-size: 1.1rem;
}
.number-input input:focus-visible { outline: none; border-color: var(--color-accent); box-shadow: 0 0 0 4px rgba(80, 70, 214, 0.16); }

/* ---------- Mapping diagram (A → f(A)) ---------- */

.mapping { width: 100%; height: auto; display: block; }
.mapping .set-shape { fill: var(--color-surface-muted); stroke: var(--color-border-strong); stroke-width: 1.5; }
.mapping .set-shape--out { fill: var(--color-accent-soft); stroke: rgba(80, 70, 214, 0.4); }
.mapping .set-title { font-family: var(--font-math); font-size: 17px; font-style: italic; fill: var(--color-text-muted); }
.mapping .elem circle { fill: var(--color-surface); stroke: var(--color-border-strong); stroke-width: 1.5; transition: fill var(--dur-fast), stroke var(--dur-fast); }
.mapping .elem text { font-family: var(--font-math); font-size: 16px; fill: var(--color-text); text-anchor: middle; dominant-baseline: central; pointer-events: none; }
.mapping .elem--in { cursor: pointer; }
.mapping .elem--in:hover circle, .mapping .elem--in:focus-visible circle { stroke: var(--color-accent); fill: var(--color-accent-soft); }
.mapping .elem--in:focus { outline: none; }
.mapping .elem--in:focus-visible circle { stroke-width: 3; }
.mapping .elem.is-done circle { fill: var(--color-accent); stroke: var(--color-accent); }
.mapping .elem.is-done text { fill: #fff; }
.mapping .elem--out.is-hit circle { fill: var(--color-accent-soft); stroke: var(--color-accent); stroke-width: 2.5; }
.mapping .arrow {
  fill: none;
  stroke: var(--color-accent);
  stroke-width: 2;
  stroke-dasharray: 300;
  stroke-dashoffset: 300;
  animation: draw-arrow 520ms var(--ease) forwards;
}
.mapping .arrow-head { fill: var(--color-accent); }
@keyframes draw-arrow { to { stroke-dashoffset: 0; } }

/* ---------- Pipeline (composition) ---------- */

.pipeline {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2);
  flex-wrap: nowrap;
}
.pipeline .machine__io { min-width: 70px; min-height: 80px; flex: 1 1 0; }
.pipeline .machine__box { min-height: 88px; flex: 1.2 1 0; padding: var(--space-3) var(--space-2); }
.pipeline .machine__box--g { background: linear-gradient(180deg, #c27a2c, #9d5f1d); box-shadow: 0 6px 18px rgba(194, 122, 44, 0.28); }
.pipeline.is-running .machine__box { animation: machine-pulse 520ms var(--ease); }
.pipeline .machine__box.is-idle { opacity: 0.55; }

.gate {
  display: grid;
  justify-items: center;
  gap: 2px;
  padding: var(--space-2) var(--space-3);
  border-radius: var(--radius-md);
  border: 2px solid var(--color-border-strong);
  font-size: var(--text-xs);
  font-weight: 700;
  color: var(--color-text-muted);
  min-width: 82px;
  text-align: center;
  transition: border-color var(--dur-med), background var(--dur-med), color var(--dur-med);
}
.gate__icon { font-size: 1.1rem; }
.gate[data-state="open"] { border-color: var(--color-success); background: var(--color-success-soft); color: var(--color-success-strong); }
.gate[data-state="closed"] { border-color: var(--color-danger); background: var(--color-danger-soft); color: var(--color-danger-strong); }

.order-callout {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-3);
  padding: var(--space-3) var(--space-4);
  border-radius: var(--radius-md);
  background: var(--color-warning-soft);
  border: 1px solid rgba(196, 140, 20, 0.35);
  color: #7a5205;
  font-weight: 700;
  letter-spacing: 0.04em;
  text-transform: uppercase;
  font-size: var(--text-sm);
}

/* ---------- Gallery ---------- */

.gallery-tabs {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: var(--space-2);
}
.gallery-tab {
  display: grid;
  gap: 2px;
  justify-items: start;
  min-height: 56px;
  padding: var(--space-2) var(--space-3);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  background: var(--color-surface);
  text-align: left;
  font-weight: 650;
  font-size: var(--text-sm);
  cursor: pointer;
  transition: border-color var(--dur-fast), background var(--dur-fast);
}
.gallery-tab small { font-family: var(--font-math); font-weight: 500; color: var(--color-text-muted); font-size: 0.95rem; }
.gallery-tab:hover { border-color: var(--color-accent); }
.gallery-tab[aria-pressed="true"] { border-color: var(--color-accent); background: var(--color-accent-soft); }
.gallery-tab.is-visited:not([aria-pressed="true"])::after { content: "✓"; justify-self: end; margin-top: -1.4em; color: var(--color-success); font-size: 0.8rem; }
.fact-grid { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: var(--space-2); }
.fact {
  padding: var(--space-2) var(--space-3);
  border-radius: var(--radius-sm);
  background: var(--color-surface-muted);
  border: 1px solid var(--color-border);
}
.fact dt { font-size: var(--text-xs); color: var(--color-text-muted); font-weight: 600; }
.fact dd { margin: 0; font-family: var(--font-math); font-size: 1.05rem; }

.domain-tag {
  display: inline-block;
  padding: 2px 10px;
  border-radius: var(--radius-pill);
  background: var(--color-accent-soft);
  color: var(--color-accent-strong);
  font-family: var(--font-math);
  font-weight: 600;
}

.two-col-table { width: 100%; border-collapse: collapse; font-family: var(--font-math); text-align: center; }
.two-col-table th { font-weight: 600; color: var(--color-text-muted); padding: var(--space-2); border-bottom: 1px solid var(--color-border); font-size: 1rem; }
.two-col-table td { padding: var(--space-2); border-bottom: 1px solid var(--color-border); font-size: 1.05rem; }
.two-col-table tr:last-child td { border-bottom: 0; }
.two-col-table td.is-undefined { color: var(--color-danger-strong); font-family: var(--font-sans); font-size: var(--text-sm); font-weight: 600; }
.two-col-table tr.is-plotted td { background: var(--color-accent-soft); }

@media (max-width: 640px) {
  .machine { grid-template-columns: minmax(0, 1fr) auto minmax(0, 1.2fr) auto minmax(0, 1fr); gap: 4px; }
  .machine__io { min-height: 78px; padding: var(--space-2) 4px; }
  .machine__value { font-size: 1.25rem; }
  .machine__rule { font-size: 0.9rem; white-space: normal; text-align: center; }
  .pipeline { flex-direction: column; align-items: stretch; }
  .pipeline > * { flex: none !important; }
  .pipeline .machine__arrow { transform: rotate(90deg); align-self: center; }
  .pipeline .machine__io, .pipeline .machine__box { min-height: 0; padding: var(--space-2); }
  .gallery-tabs { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .options--inline { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .fact-grid { grid-template-columns: minmax(0, 1fr); }
}

/* =========================================================
   Components added for 1.3 Μονότονες συναρτήσεις – Αντίστροφη
   ========================================================= */

.ineq-display--pair {
  flex-wrap: wrap;
  gap: var(--space-2) var(--space-3);
  font-size: clamp(1rem, 0.9rem + 0.5vw, 1.25rem);
  padding: var(--space-3);
}
.ineq-display__rel.is-up { color: var(--color-success-strong); }
/* Canvases whose handle is dragged vertically must receive vertical touch moves (sliders stay as the alternative). */
.graph.graph--drag-y { touch-action: none; }

.mini-graphs { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: var(--space-4); }
.mini-graphs--two { grid-template-columns: repeat(2, minmax(0, 1fr)); gap: var(--space-3); }
.mini-graph { display: grid; gap: var(--space-3); align-content: start; border: 2px solid var(--color-border); transition: border-color var(--dur-med), background var(--dur-med); }
.mini-graph.is-correct { border-color: var(--color-success); background: var(--color-success-soft); }
.mini-graph.is-wrong { border-color: var(--color-danger); animation: nudge 360ms var(--ease); }
.mini-graph.is-correct .interval-card__hint { color: var(--color-success-strong); }
.mini-figure { margin: 0; display: grid; gap: var(--space-1); }
.mini-figure figcaption { font-size: var(--text-xs); color: var(--color-text-muted); text-align: center; }

.choice-group--row { grid-template-columns: repeat(3, minmax(0, 1fr)); }
.choice-group--row .choice { padding: var(--space-2) 4px; }

.mapping .arrow--clash { stroke: var(--color-danger); }
.mapping .elem--out.is-clash circle { stroke: var(--color-danger); stroke-width: 3; fill: var(--color-danger-soft); }

.machine__box--reverse { background: linear-gradient(180deg, #2b3a67, var(--color-text)); box-shadow: 0 6px 18px rgba(21, 32, 59, 0.28); }

.swap-sets {
  display: grid;
  grid-template-columns: minmax(0, 1fr) auto minmax(0, 1fr);
  align-items: center;
  gap: var(--space-3);
}
.swap-set {
  padding: var(--space-4);
  border-radius: var(--radius-md);
  text-align: center;
  border: 1.5px solid var(--color-border-strong);
  background: var(--color-surface-muted);
}
.swap-set--right { background: var(--color-accent-soft); border-color: rgba(80, 70, 214, 0.35); }
.swap-set__role { font-size: var(--text-xs); font-weight: 700; letter-spacing: 0.05em; text-transform: uppercase; color: var(--color-text-muted); }
.swap-set__value { font-size: 1.25rem; margin-top: var(--space-1); }
.swap-arrow { display: grid; justify-items: center; font-size: 1.3rem; color: var(--color-accent-strong); }
.swap-sets.is-swapping .swap-set--left { animation: swap-left var(--dur-slow) var(--ease); }
.swap-sets.is-swapping .swap-set--right { animation: swap-right var(--dur-slow) var(--ease); }
@keyframes swap-left { from { transform: translateX(60%); opacity: 0.3; } to { transform: none; opacity: 1; } }
@keyframes swap-right { from { transform: translateX(-60%); opacity: 0.3; } to { transform: none; opacity: 1; } }

@media (max-width: 640px) {
  .mini-graphs { grid-template-columns: minmax(0, 1fr); }
  .mini-graphs--two { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .swap-sets { grid-template-columns: minmax(0, 1fr); }
  .swap-arrow { transform: rotate(90deg); }
}

/* =========================================================
   Components added for 1.4 Όριο συνάρτησης στο x₀
   ========================================================= */

/* One question row inside a mini graph card (left limit / right limit / exists?) */
.limit-row { display: grid; gap: var(--space-1); }
.limit-row__label {
  font-size: var(--text-xs);
  font-weight: 700;
  letter-spacing: 0.04em;
  color: var(--color-text-muted);
  text-align: center;
}
.stage__text .limit-row__label { color: var(--color-text-muted); }
.choice-group--pair { grid-template-columns: repeat(2, minmax(0, 1fr)); }
.limit-row .choice { font-family: var(--font-math); font-size: 0.95rem; line-height: 1.2; }
.limit-row .choice[data-choice="nd"],
.limit-row .choice-group--pair .choice { font-family: var(--font-sans); font-size: var(--text-xs); }

/* Numbered flow that lights up step by step (ε → δ → x → check) */
.flow--steps { list-style: none; margin: 0; padding: 0; gap: var(--space-2); }
.flow--steps .flow__card {
  padding: var(--space-3) var(--space-4);
  transition: border-color var(--dur-med), background var(--dur-med), box-shadow var(--dur-med);
}
.flow--steps .flow__title { font-weight: 600; }
.flow__card.is-lit {
  border-color: var(--color-accent);
  background: var(--color-accent-soft);
  box-shadow: 0 0 0 3px rgba(80, 70, 214, 0.14);
}
.flow__card.is-done .flow__num { background: var(--color-success); }

/* Rounds of the ε–δ game */
.rounds { list-style: none; margin: 0; padding: 0; display: grid; gap: var(--space-2); }
.rounds li {
  padding: var(--space-2) var(--space-4);
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-md);
  font-size: var(--text-sm);
  font-weight: 600;
  color: var(--color-text-muted);
}
.rounds li.is-current { border-color: var(--color-accent); background: var(--color-accent-soft); color: var(--color-accent-strong); }
.rounds li.is-done { border-color: var(--color-success); background: var(--color-success-soft); color: var(--color-success-strong); }
.rounds li.is-done::after { content: " ✓"; }

.options--quad { grid-template-columns: repeat(2, minmax(0, 1fr)); }
.live-check--wrap { white-space: normal; line-height: 1.7; }
.graph-card.is-success { border-color: var(--color-success); box-shadow: 0 0 0 3px rgba(30, 148, 103, 0.16), var(--shadow-md); }
.meaning .btn--ghost { margin-top: var(--space-2); }

/* =========================================================
   Components added for 1.5 Ιδιότητες των ορίων
   ========================================================= */

/* Step-by-step solver: derivation lines + "what comes next?" choices */
.stepper { display: grid; gap: var(--space-4); }
.stepper .derivation__line { flex-wrap: wrap; }
.stepper .derivation__line .formula { white-space: normal; line-height: 1.9; }
.step-ask {
  padding: var(--space-4);
  border-radius: var(--radius-md);
  border: 1.5px dashed rgba(80, 70, 214, 0.45);
  background: var(--color-surface);
}
.step-ask .step-counter { margin-bottom: var(--space-1); }
.step-ask__prompt { font-weight: 600; margin-bottom: var(--space-3); color: var(--color-text); }
.step-ask__prompt:focus { outline: none; }
.step-ask__choices .choice { font-family: var(--font-math); font-size: 1rem; line-height: 1.5; }
.step-ask__hint { margin-top: var(--space-2); text-align: left; font-size: var(--text-sm); }
.rule-tag {
  margin-left: auto;
  padding: 2px var(--space-2);
  border-radius: var(--radius-pill);
  background: var(--color-surface);
  border: 1px solid rgba(80, 70, 214, 0.3);
  color: var(--color-accent-strong);
  font-size: var(--text-xs);
  font-weight: 700;
  white-space: nowrap;
}
.muted-note { font-family: var(--font-sans); font-size: var(--text-sm); color: var(--color-text-muted); }

/* Toolbox of limit rules */
.toolbox { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: var(--space-2); }
.tool-card {
  display: grid;
  justify-items: center;
  gap: 2px;
  min-height: 64px;
  padding: var(--space-2);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  background: var(--color-surface-muted);
  font-size: var(--text-sm);
  font-weight: 600;
  color: var(--color-text);
  cursor: pointer;
  transition: border-color var(--dur-fast), background var(--dur-fast);
}
.tool-card:hover { border-color: var(--color-accent); }
.tool-card__icon { font-family: var(--font-math); font-size: 1.2rem; color: var(--color-accent-strong); }
.tool-card.is-visited { border-color: rgba(80, 70, 214, 0.35); }
.tool-card[aria-pressed="true"] { border-color: var(--color-accent); background: var(--color-accent-soft); box-shadow: 0 0 0 3px rgba(80, 70, 214, 0.12); }
.tool-detail {
  display: grid;
  gap: var(--space-3);
  padding: var(--space-4);
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
  background: var(--color-surface);
}
.tool-detail .formula-card__label { margin-bottom: 0; }
.tool-detail .formula { font-size: 1.15rem; line-height: 2; }
.tool-detail .calc { font-family: var(--font-math); font-size: 1.1rem; }

/* Summary: strategy flowchart + rule chips */
.strategy { display: grid; gap: var(--space-3); margin-bottom: var(--space-5); }
.strategy__node--start {
  justify-self: center;
  padding: var(--space-3) var(--space-5);
  border-radius: var(--radius-pill);
  background: var(--color-accent);
  color: #fff;
  font-weight: 700;
  text-align: center;
}
.strategy__split { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: var(--space-3); }
.strategy__split--three { grid-template-columns: repeat(3, minmax(0, 1fr)); }
.strategy__branch {
  display: grid;
  align-content: start;
  gap: var(--space-2);
  padding: var(--space-4);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  background: var(--color-surface-muted);
  font-size: var(--text-sm);
}
.strategy__tag {
  justify-self: start;
  padding: 2px var(--space-3);
  border-radius: var(--radius-pill);
  background: var(--color-accent-soft);
  color: var(--color-accent-strong);
  font-size: var(--text-xs);
  font-weight: 700;
  letter-spacing: 0.04em;
}
.strategy__tag--yes { background: var(--color-success-soft); color: var(--color-success-strong); }
.strategy__tag--no { background: var(--color-danger-soft); color: var(--color-danger-strong); }
.strategy__list { margin: 0; padding-left: var(--space-5); }
.rule-chips { display: flex; flex-wrap: wrap; gap: var(--space-2); margin: 0 0 var(--space-5); padding: 0; list-style: none; }
.rule-chips li {
  padding: var(--space-2) var(--space-3);
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
  background: var(--color-surface-muted);
  font-size: 0.95rem;
}

/* =========================================================
   Components added for 1.6 Μη πεπερασμένο όριο στο x₀
   ========================================================= */

.mini-graphs--stack { grid-template-columns: minmax(0, 1fr); gap: var(--space-2); }
.segmented--wrap { display: flex; flex-wrap: wrap; }
.segmented--wrap button { flex: 1 1 auto; }
.segmented button.is-done::after { content: " ✓"; color: var(--color-success-strong); }
.form-cards { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: var(--space-2); }
.form-card {
  display: grid;
  place-items: center;
  min-height: 64px;
  padding: var(--space-2);
  border-radius: var(--radius-md);
  border: 1.5px solid rgba(212, 67, 79, 0.35);
  background: var(--color-danger-soft);
  color: var(--color-danger-strong);
  font-family: var(--font-math);
  font-size: 1.15rem;
  font-weight: 600;
}
.classify { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: var(--space-3); }
.classify .limit-row {
  padding: var(--space-3);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  background: var(--color-surface);
}
.classify .limit-row__label { font-size: 1.05rem; color: var(--color-text); letter-spacing: 0; }

@media (max-width: 640px) {
  .form-cards { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .classify { grid-template-columns: minmax(0, 1fr); }
  .toolbox { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .strategy__split, .strategy__split--three { grid-template-columns: minmax(0, 1fr); }
  .rule-tag { margin-left: 0; }
}

@media (max-width: 480px) {
  .options--inline { grid-template-columns: minmax(0, 1fr); }
}

/* ---------- Reduced motion ---------- */

@media (prefers-reduced-motion: reduce) {
  *,
  *::before,
  *::after {
    animation-duration: 1ms !important;
    animation-delay: 0ms !important;
    transition-duration: 1ms !important;
    scroll-behavior: auto !important;
  }
}