:root {
  --ink: #18231f;
  --muted: #68736d;
  --paper: #f4f1e8;
  --panel: #fbfaf5;
  --line: #d9d5c9;
  --green: #174f42;
  --green-2: #236b59;
  --lime: #d9f16f;
  --coral: #f05d3a;
  --shadow: 0 22px 70px rgba(30, 46, 39, .12);
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  color: var(--ink);
  background:
    radial-gradient(circle at 88% 8%, rgba(217, 241, 111, .22), transparent 24rem),
    var(--paper);
  font-family: Inter, "Noto Sans SC", "Microsoft YaHei", system-ui, sans-serif;
  line-height: 1.65;
}

button, input { font: inherit; }
button, a { -webkit-tap-highlight-color: transparent; }

.topbar {
  position: sticky;
  top: 0;
  z-index: 30;
  display: flex;
  align-items: center;
  justify-content: space-between;
  min-height: 68px;
  padding: 0 clamp(22px, 5vw, 76px);
  background: rgba(244, 241, 232, .88);
  border-bottom: 1px solid rgba(24, 35, 31, .1);
  backdrop-filter: blur(18px);
}

.brand {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  color: var(--ink);
  font-weight: 750;
  text-decoration: none;
  letter-spacing: .03em;
}

.brand-mark {
  display: grid;
  width: 32px;
  height: 32px;
  place-items: center;
  color: var(--lime);
  background: var(--green);
  border-radius: 9px;
  font-size: 17px;
}

.topbar nav { display: flex; gap: 28px; }
.topbar nav a { color: #42504a; font-size: 14px; text-decoration: none; }
.topbar nav a:hover { color: var(--green); }

.hero {
  width: min(1180px, calc(100% - 44px));
  margin: 0 auto;
  padding: clamp(72px, 11vw, 138px) 0 82px;
}

.eyebrow, .section-kicker {
  margin: 0 0 18px;
  color: var(--green-2);
  font-size: 12px;
  font-weight: 800;
  letter-spacing: .16em;
  text-transform: uppercase;
}

.eyebrow { display: flex; align-items: center; gap: 9px; }
.eyebrow span { width: 24px; height: 2px; background: var(--coral); }

h1 {
  max-width: 900px;
  margin: 0;
  font-family: "STSong", "Songti SC", Georgia, serif;
  font-size: clamp(46px, 7.4vw, 92px);
  font-weight: 650;
  letter-spacing: -.045em;
  line-height: 1.08;
}

h1 em { color: var(--coral); font-style: normal; }

.hero-lead {
  max-width: 720px;
  margin: 30px 0 0;
  color: #52605a;
  font-size: clamp(16px, 1.7vw, 20px);
}

.hero-actions { display: flex; flex-wrap: wrap; align-items: center; gap: 14px; margin-top: 38px; }
.primary-action, .image-button {
  min-height: 50px;
  border: 0;
  border-radius: 999px;
  cursor: pointer;
  font-weight: 750;
  text-decoration: none;
}

.primary-action {
  display: inline-flex;
  align-items: center;
  gap: 28px;
  padding: 0 24px;
  color: #fff;
  background: var(--green);
  box-shadow: 0 8px 24px rgba(23, 79, 66, .2);
}

.primary-action:hover { background: #0e3d33; transform: translateY(-1px); }
.image-button { padding: 0 22px; color: var(--ink); background: transparent; border: 1px solid var(--line); }
.image-button:hover { background: rgba(255,255,255,.6); }

.fact-row {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  margin-top: clamp(65px, 10vw, 110px);
  border-top: 1px solid var(--line);
  border-bottom: 1px solid var(--line);
}

.fact-row div { display: flex; flex-direction: column; padding: 22px 24px 22px 0; }
.fact-row div + div { padding-left: 26px; border-left: 1px solid var(--line); }
.fact-row strong { font-family: Georgia, serif; font-size: 34px; line-height: 1; }
.fact-row span { margin-top: 7px; color: var(--muted); font-size: 13px; }
.fact-row .answer-fact strong { color: var(--coral); }

.experiment-section {
  padding: 82px clamp(18px, 4vw, 60px) 100px;
  color: #eef6f0;
  background: var(--green);
}

.section-heading {
  display: flex;
  align-items: end;
  justify-content: space-between;
  width: min(1320px, 100%);
  margin: 0 auto 30px;
}

.section-heading .section-kicker { color: var(--lime); }
.section-heading h2, .reasoning-intro h2 {
  margin: 0;
  font-family: "STSong", "Songti SC", Georgia, serif;
  font-size: clamp(34px, 4vw, 54px);
  line-height: 1.15;
}
.section-heading > p { margin: 0 0 8px; color: rgba(238,246,240,.66); font-size: 13px; }

.lab-shell {
  display: grid;
  grid-template-columns: 340px minmax(0, 1fr);
  width: min(1320px, 100%);
  min-height: 760px;
  margin: 0 auto;
  overflow: hidden;
  background: #f8f6ef;
  border-radius: 24px;
  box-shadow: 0 35px 90px rgba(7, 29, 24, .35);
}

.lesson-rail {
  position: relative;
  z-index: 2;
  padding: 28px 25px 24px;
  color: var(--ink);
  background: #fbfaf5;
  border-right: 1px solid var(--line);
}

.progress-label { display: flex; justify-content: space-between; color: var(--muted); font-size: 12px; }
.progress-label b { color: var(--green); }
.progress-track { height: 3px; margin: 10px 0 22px; overflow: hidden; background: #e1dfd5; border-radius: 2px; }
.progress-track span { display: block; width: 25%; height: 100%; background: var(--coral); transition: width .35s ease; }

.step-list { display: grid; gap: 5px; }
.step-button {
  display: grid;
  grid-template-columns: 42px 1fr;
  gap: 4px;
  width: 100%;
  padding: 12px 10px;
  color: #61706a;
  text-align: left;
  background: transparent;
  border: 0;
  border-radius: 12px;
  cursor: pointer;
}
.step-button:hover { background: #f0eee6; }
.step-button.is-active { color: var(--ink); background: #ece9df; }
.step-number { padding-top: 1px; color: #97a09b; font-family: Georgia, serif; font-size: 14px; }
.step-button.is-active .step-number { color: var(--coral); }
.step-button b, .step-button small { display: block; }
.step-button b { font-size: 14px; }
.step-button small { margin-top: 1px; color: #84908a; font-size: 11px; }

.lesson-note {
  min-height: 92px;
  margin: 20px 2px 15px;
  padding: 14px 15px;
  color: #415049;
  background: #eef3e8;
  border-left: 3px solid var(--lime);
  border-radius: 0 10px 10px 0;
  font-size: 12.5px;
  line-height: 1.65;
}
.lesson-note strong { color: var(--green); }

.control-group { margin-top: 17px; padding-top: 16px; border-top: 1px solid var(--line); }
.control-heading, .slider-label { display: flex; align-items: center; justify-content: space-between; color: #59665f; font-size: 12px; font-weight: 700; }
.control-heading button { color: var(--green); background: none; border: 0; cursor: pointer; font-size: 11px; }
.family-buttons { display: grid; grid-template-columns: 1fr 1fr; gap: 7px; margin: 10px 0 14px; }
.family-buttons button, .layer-filters button {
  color: #5a6761;
  background: #f3f1e9;
  border: 1px solid #dedbd0;
  border-radius: 9px;
  cursor: pointer;
  font-size: 11px;
}
.family-buttons button { display: flex; align-items: center; gap: 6px; padding: 7px 9px; }
.family-buttons button i { width: 7px; height: 7px; background: var(--family); border-radius: 50%; }
.family-buttons button.is-active { color: var(--ink); background: #fff; border-color: #a9aaa2; box-shadow: 0 3px 10px rgba(20,35,29,.07); }

input[type="range"] {
  width: 100%;
  height: 4px;
  margin: 13px 0 7px;
  accent-color: var(--coral);
}

.layer-filters { display: grid; grid-template-columns: repeat(4, 1fr); gap: 5px; margin-top: 10px; }
.layer-filters button { padding: 6px 2px; }
.layer-filters button.is-active { color: #fff; background: var(--green); border-color: var(--green); }

.view-actions {
  position: absolute;
  right: 25px;
  bottom: 20px;
  left: 25px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding-top: 14px;
  color: #69756f;
  border-top: 1px solid var(--line);
  font-size: 11px;
}
.view-actions label { display: flex; align-items: center; gap: 4px; cursor: pointer; }
.view-actions input { accent-color: var(--green); }
.view-actions button { padding: 4px 0; color: var(--green); background: none; border: 0; cursor: pointer; font-weight: 700; }

.viewer-card { position: relative; min-width: 0; min-height: 760px; overflow: hidden; background: #e8e5dc; }
.viewer-card::before {
  position: absolute;
  inset: 0;
  content: "";
  pointer-events: none;
  background:
    linear-gradient(rgba(23,79,66,.045) 1px, transparent 1px),
    linear-gradient(90deg, rgba(23,79,66,.045) 1px, transparent 1px);
  background-size: 34px 34px;
  mask-image: radial-gradient(circle, black 20%, transparent 74%);
}

#sceneCanvas { position: absolute; inset: 0; display: block; width: 100%; height: 100%; cursor: grab; touch-action: none; }
#sceneCanvas:active { cursor: grabbing; }

.viewer-status {
  position: absolute;
  top: 20px;
  left: 22px;
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 7px 12px;
  color: var(--green);
  background: rgba(251,250,245,.84);
  border: 1px solid rgba(23,79,66,.14);
  border-radius: 999px;
  backdrop-filter: blur(9px);
  font-size: 11px;
  font-weight: 800;
}
.status-dot { width: 7px; height: 7px; background: var(--coral); border-radius: 50%; box-shadow: 0 0 0 4px rgba(240,93,58,.14); }

.near-vertex-badge {
  --near-color: #f05d3a;
  position: absolute;
  top: 62px;
  left: 22px;
  display: flex;
  align-items: center;
  gap: 6px;
  padding: 6px 10px;
  color: #53615b;
  background: rgba(251,250,245,.78);
  border: 1px solid rgba(23,79,66,.1);
  border-radius: 8px;
  backdrop-filter: blur(8px);
  font-size: 10px;
  pointer-events: none;
}
.near-vertex-badge[hidden] { display: none; }
.near-vertex-badge span { width: 7px; height: 7px; background: var(--near-color); border-radius: 50%; box-shadow: 0 0 0 4px color-mix(in srgb, var(--near-color) 18%, transparent); }
.near-vertex-badge b { color: var(--near-color); font-size: 12px; }

.axis-legend { position: absolute; top: 20px; right: 22px; display: flex; gap: 7px; }
.axis-legend span {
  display: grid;
  width: 28px;
  height: 28px;
  place-items: center;
  color: #fff;
  background: var(--family);
  border: 2px solid rgba(255,255,255,.76);
  border-radius: 50%;
  box-shadow: 0 4px 13px rgba(27,39,34,.14);
  font-size: 11px;
  font-weight: 800;
}

.gesture-hint {
  position: absolute;
  bottom: 22px;
  left: 22px;
  display: flex;
  align-items: center;
  gap: 8px;
  color: #6c7772;
  font-size: 11px;
  transition: opacity .25s;
}
.gesture-hint span { display: grid; width: 27px; height: 27px; place-items: center; background: rgba(255,255,255,.66); border-radius: 50%; }

.next-step {
  position: absolute;
  right: 22px;
  bottom: 18px;
  display: flex;
  align-items: center;
  gap: 20px;
  padding: 10px 15px;
  color: #fff;
  background: var(--green);
  border: 0;
  border-radius: 11px;
  box-shadow: 0 7px 18px rgba(23,79,66,.2);
  cursor: pointer;
  font-size: 11px;
  font-weight: 750;
}
.next-step:hover { transform: translateY(-1px); }

.canvas-tooltip {
  position: absolute;
  z-index: 4;
  max-width: 210px;
  padding: 9px 11px;
  color: #fff;
  background: rgba(18, 37, 31, .92);
  border-radius: 8px;
  box-shadow: 0 8px 28px rgba(0,0,0,.18);
  pointer-events: none;
  font-size: 11px;
  line-height: 1.45;
}
.canvas-tooltip strong { display: block; color: var(--lime); font-size: 12px; }

.selection-readout {
  grid-template-columns: 150px minmax(270px, 1fr) minmax(250px, 1.2fr);
  align-items: center;
  gap: 26px;
  width: min(1320px, 100%);
  margin: 16px auto 0;
  padding: 20px 24px;
  color: var(--ink);
  background: #f8f6ef;
  border-radius: 14px;
}
.selection-readout:not([hidden]) { display: grid; }
.readout-kicker { display: block; color: var(--muted); font-size: 10px; letter-spacing: .1em; }
.selection-readout strong { font-family: Georgia, serif; font-size: 20px; }
.tuple { display: flex; gap: 7px; }
.tuple span { display: grid; min-width: 54px; padding: 5px 8px; place-items: center; color: #fff; border-radius: 7px; font-size: 12px; font-weight: 750; }
.readout-copy { padding-left: 22px; border-left: 1px solid var(--line); }
.readout-copy span { color: var(--coral); font-weight: 800; font-size: 12px; }
.readout-copy p { margin: 3px 0 0; color: #637069; font-size: 11px; }

.reasoning-section { width: min(1180px, calc(100% - 44px)); margin: 0 auto; padding: 108px 0 100px; }
.reasoning-intro { max-width: 830px; }
.reasoning-intro .section-kicker { color: var(--coral); }
.reasoning-intro p:last-child { margin: 25px 0 0; color: #56635d; font-size: 17px; }
.math { font-family: Georgia, "Times New Roman", serif; color: var(--green); }

.dedupe-diagram {
  display: grid;
  grid-template-columns: 1fr auto 1fr auto 1fr;
  align-items: center;
  gap: 18px;
  margin: 62px 0;
  padding: 26px;
  background: #eae7de;
  border-radius: 18px;
}
.diagram-node { display: flex; flex-direction: column; padding: 15px 18px; background: rgba(255,255,255,.55); border-radius: 12px; }
.diagram-node strong { color: var(--green); font-family: Georgia, serif; font-size: clamp(21px, 2.5vw, 29px); }
.diagram-node span { color: var(--muted); font-size: 11px; }
.diagram-node.accent { background: var(--green); }
.diagram-node.accent strong { color: var(--lime); }
.diagram-node.accent span { color: rgba(255,255,255,.72); }
.diagram-arrow { display: flex; flex-direction: column; align-items: center; color: var(--coral); font-size: 22px; }
.diagram-arrow span { color: #7b8580; font-size: 9px; white-space: nowrap; }

.proof-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 18px; }
.proof-card { padding: 30px; background: var(--panel); border: 1px solid var(--line); border-radius: 16px; }
.card-index { color: var(--coral); font-size: 10px; font-weight: 800; letter-spacing: .14em; }
.proof-card h3 { margin: 7px 0 14px; font-family: "STSong", "Songti SC", Georgia, serif; font-size: 25px; }
.proof-card p { margin: 8px 0; color: #5e6a64; font-size: 14px; }
.equation { margin: 19px 0; padding: 18px; color: var(--green); background: #ebf0e7; border-radius: 10px; font-family: Georgia, serif; font-size: clamp(18px, 2vw, 24px); text-align: center; }
.equation.compact { font-size: clamp(15px, 1.7vw, 20px); }

.counting-row { display: grid; grid-template-columns: 1fr auto 1fr auto 1fr auto 1.1fr; align-items: stretch; gap: 12px; margin-top: 44px; }
.counting-row article { position: relative; display: flex; flex-direction: column; justify-content: center; min-height: 150px; padding: 22px; overflow: hidden; background: #ece9df; border-radius: 14px; }
.counting-row article p, .counting-row article small { margin: 0; color: var(--muted); }
.counting-row article strong { color: var(--green); font-family: Georgia, serif; font-size: clamp(18px, 2vw, 26px); }
.counting-row article small { font-size: 10px; }
.layer-swatch { position: absolute; top: 0; right: 0; left: 0; height: 5px; }
.s2 { background: #ef6b48; }
.s3 { background: #e6ae3f; }
.s4 { background: #3f9b81; }
.counting-row .plus, .counting-row .equals { align-self: center; color: #9ba29e; font-family: Georgia, serif; font-size: 28px; }
.counting-row .total-card { color: #fff; background: var(--green); }
.counting-row .total-card p, .counting-row .total-card small { color: rgba(255,255,255,.65); }
.counting-row .total-card strong { color: var(--lime); font-size: 50px; line-height: 1.1; }

.conclusion { display: grid; grid-template-columns: 190px 1fr; align-items: center; gap: 36px; margin-top: 70px; padding-top: 36px; border-top: 1px solid var(--line); }
.conclusion > div { display: flex; align-items: center; gap: 14px; }
.conclusion > div span { color: var(--muted); font-size: 12px; }
.conclusion > div strong { color: var(--coral); font-family: Georgia, serif; font-size: 58px; }
.conclusion p { margin: 0; color: #56625c; font-size: 17px; }

footer { display: flex; justify-content: space-between; padding: 26px clamp(22px, 5vw, 76px); color: #b7c5be; background: #113b31; font-size: 11px; }
footer span { color: #fff; font-weight: 750; }
footer p { margin: 0; }

dialog {
  width: min(1000px, calc(100% - 30px));
  padding: 0;
  overflow: hidden;
  background: var(--panel);
  border: 0;
  border-radius: 18px;
  box-shadow: 0 30px 100px rgba(0,0,0,.32);
}
dialog::backdrop { background: rgba(12,29,24,.72); backdrop-filter: blur(7px); }
dialog img { display: block; width: 100%; height: auto; }
.dialog-bar { display: flex; align-items: center; justify-content: space-between; padding: 13px 17px; border-bottom: 1px solid var(--line); }
.dialog-bar div { display: flex; align-items: baseline; gap: 9px; }
.dialog-bar span { color: var(--muted); font-size: 11px; }
.dialog-bar button { width: 32px; height: 32px; color: var(--muted); background: #eceae2; border: 0; border-radius: 50%; cursor: pointer; font-size: 21px; line-height: 1; }

@media (max-width: 980px) {
  .lab-shell { grid-template-columns: 290px minmax(0, 1fr); }
  .lesson-rail { padding-right: 18px; padding-left: 18px; }
  .view-actions { right: 18px; left: 18px; }
  .selection-readout { grid-template-columns: 120px 1fr; }
  .readout-copy { grid-column: 1 / -1; padding: 12px 0 0; border-top: 1px solid var(--line); border-left: 0; }
  .counting-row { grid-template-columns: 1fr 1fr 1fr; }
  .counting-row .plus, .counting-row .equals { display: none; }
  .counting-row .total-card { grid-column: 1 / -1; min-height: 110px; }
}

@media (max-width: 760px) {
  .topbar { min-height: 58px; }
  .topbar nav { display: none; }
  .hero { padding-top: 68px; }
  .fact-row { grid-template-columns: 1fr 1fr; }
  .fact-row div:nth-child(3) { border-left: 0; border-top: 1px solid var(--line); }
  .fact-row div:nth-child(4) { border-top: 1px solid var(--line); }
  .section-heading { align-items: start; }
  .section-heading > p { display: none; }
  .experiment-section { padding-right: 12px; padding-left: 12px; }
  .lab-shell { display: flex; flex-direction: column; }
  .viewer-card { order: -1; min-height: 480px; }
  .lesson-rail { min-height: 590px; border-top: 1px solid var(--line); border-right: 0; }
  .step-list { grid-template-columns: 1fr 1fr; }
  .step-button { grid-template-columns: 32px 1fr; padding: 10px 7px; }
  .step-button small { display: none; }
  .view-actions { bottom: 16px; }
  .selection-readout { grid-template-columns: 1fr; }
  .tuple { flex-wrap: wrap; }
  .dedupe-diagram { grid-template-columns: 1fr; }
  .diagram-arrow { transform: rotate(90deg); }
  .diagram-arrow span { display: none; }
  .proof-grid { grid-template-columns: 1fr; }
  .counting-row { grid-template-columns: 1fr; }
  .conclusion { grid-template-columns: 1fr; gap: 8px; }
}

@media (max-width: 480px) {
  .hero { width: min(100% - 30px, 1180px); }
  h1 { font-size: 44px; }
  .hero-lead { font-size: 15px; }
  .primary-action { width: 100%; justify-content: space-between; }
  .fact-row div { padding: 17px 10px 17px 0; }
  .fact-row div + div { padding-left: 16px; }
  .viewer-card { min-height: 410px; }
  .lesson-rail { min-height: 620px; }
  .next-step { right: 14px; bottom: 14px; }
  .gesture-hint { display: none; }
  .axis-legend { right: 14px; }
  .near-vertex-badge { top: 56px; left: 14px; }
  .axis-legend span { width: 25px; height: 25px; }
  .selection-readout { padding: 18px; }
  .reasoning-section { width: min(100% - 30px, 1180px); padding-top: 76px; }
  .dedupe-diagram { padding: 18px; }
  .proof-card { padding: 22px; }
  footer { display: block; }
  footer p { margin-top: 4px; }
}

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  *, *::before, *::after { transition-duration: .001ms !important; animation-duration: .001ms !important; }
}