:root {
  --ink: #232b32;
  --ink-soft: #3f4b54;
  --muted: #66737d;
  --paper: #ffffff;
  --ground: #eef1f2;
  --canvas: #f7f8f8;
  --line: #d7dde0;
  --line-strong: #aeb9bf;
  --blue: #2c67a0;
  --blue-dark: #1d4f7f;
  --blue-soft: #e4eef7;
  --rust: #bd5f35;
  --rust-soft: #f5e8e0;
  --teal: #28756e;
  --teal-soft: #e4f0ee;
  --red: #a74943;
  --red-soft: #f5e6e4;
  --sans: "Avenir Next", "Segoe UI", ui-sans-serif, system-ui, sans-serif;
  --mono: "SFMono-Regular", Consolas, "Liberation Mono", monospace;
  --shadow: 0 16px 40px rgba(35, 43, 50, 0.07);
}

*, *::before, *::after { box-sizing: border-box; }
html { color-scheme: light; scroll-behavior: smooth; }
body {
  min-width: 320px;
  margin: 0;
  background: var(--ground);
  color: var(--ink);
  font-family: var(--sans);
  font-size: 16px;
  line-height: 1.5;
}

button, input, select { font: inherit; }
button { color: inherit; }
a { color: var(--blue-dark); }
[hidden] { display: none !important; }

.skip-link {
  position: fixed;
  top: 8px;
  left: 8px;
  z-index: 4;
  padding: 8px 12px;
  background: var(--paper);
  border: 2px solid var(--blue);
  transform: translateY(-150%);
}
.skip-link:focus { transform: translateY(0); }

.topbar {
  min-height: 64px;
  display: grid;
  grid-template-columns: minmax(130px, 1fr) auto minmax(250px, 1fr);
  align-items: center;
  gap: 24px;
  padding: 12px clamp(18px, 3vw, 44px);
  border-bottom: 4px solid var(--rust);
  background: var(--ink);
  color: #fff;
}
.back-link,
.top-links a {
  color: #dbe2e6;
  font-size: 0.84rem;
  font-weight: 700;
  text-decoration: none;
}
.back-link::before { content: "← "; }
.back-link:hover,
.top-links a:hover { color: #fff; text-decoration: underline; text-underline-offset: 4px; }
.course-mark { display: grid; justify-items: center; line-height: 1.15; }
.course-mark span { color: #bfcbd1; font: 750 0.66rem var(--mono); letter-spacing: 0.1em; text-transform: uppercase; }
.course-mark strong { margin-top: 4px; font-size: 1.28rem; letter-spacing: -0.02em; }
.top-links { display: flex; justify-content: flex-end; gap: 20px; }

.app { max-width: 1540px; margin: 0 auto; padding: 7px clamp(12px, 2.2vw, 32px) 38px; }
.intro {
  display: grid;
  grid-template-columns: minmax(0, 1.35fr) minmax(340px, 0.65fr);
  gap: clamp(28px, 5vw, 76px);
  align-items: end;
  padding: 2px 0 12px;
}
.eyebrow {
  margin: 0 0 4px;
  color: var(--blue);
  font: 800 0.69rem var(--mono);
  letter-spacing: 0.105em;
  text-transform: uppercase;
}
h1, h2, h3, p { overflow-wrap: break-word; }
h1 { max-width: 21ch; margin: 0; font-size: clamp(2rem, 3vw, 3.05rem); line-height: 1; letter-spacing: -0.05em; }
.lead { max-width: 72ch; margin: 8px 0 0; color: var(--ink-soft); font-size: clamp(0.92rem, 1.05vw, 1.04rem); }
.theorem-card {
  padding: 11px 14px;
  border: 1px solid var(--line-strong);
  border-top: 4px solid var(--ink);
  background: rgba(255, 255, 255, 0.72);
  box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.85);
}
.theorem-card p { margin: 0; }
.theorem-label { margin-bottom: 4px !important; color: var(--rust); font: 800 0.67rem var(--mono); letter-spacing: 0.08em; text-transform: uppercase; }
.theorem-math { margin: 5px 0 !important; color: var(--ink); font: 700 0.94rem var(--mono); }
.theorem-card > p:not(.theorem-label):not(.theorem-math):not(.theorem-caveat) { font-size: 0.78rem; }
.theorem-caveat { padding-top: 6px; border-top: 1px solid var(--line); color: var(--muted); font-size: 0.7rem; }

.lane-tabs {
  width: min(720px, 100%);
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  margin: 0 0 10px;
  padding: 4px;
  border: 1px solid var(--line-strong);
  background: #dde3e6;
}
.lane-tab {
  min-height: 54px;
  display: grid;
  grid-template-columns: auto 1fr;
  align-items: center;
  gap: 11px;
  padding: 9px 14px;
  border: 0;
  background: transparent;
  text-align: left;
  cursor: pointer;
  transition: background 180ms ease, transform 180ms ease;
}
.lane-tab:hover { background: rgba(255, 255, 255, 0.65); }
.lane-tab:active { transform: translateY(1px); }
.lane-tab.is-active { background: var(--paper); box-shadow: 0 3px 10px rgba(35, 43, 50, 0.1); }
.lane-tab strong, .lane-tab small { display: block; }
.lane-tab strong { font-size: 0.9rem; }
.lane-tab small { margin-top: 1px; color: var(--muted); font-size: 0.72rem; }
.lane-index { width: 30px; height: 30px; display: grid; place-items: center; border: 1px solid currentColor; font: 750 0.77rem var(--mono); }
.lane-tab.is-active .lane-index { border-color: var(--blue); background: var(--blue); color: #fff; }

.lab-shell { display: grid; grid-template-columns: 390px minmax(0, 1fr); align-items: start; gap: 18px; }
.controls,
.workspace { border: 1px solid var(--line); background: var(--paper); box-shadow: var(--shadow); }
.controls { position: sticky; top: 12px; display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); align-items: start; }
.control-group { padding: 12px 15px; border-bottom: 1px solid var(--line); }
.control-group:last-child { border-bottom: 0; }
.action-group { grid-column: 1 / -1; }
.section-heading { display: flex; align-items: center; gap: 9px; margin-bottom: 9px; }
.section-heading > span { width: 23px; height: 23px; display: grid; place-items: center; border: 1px solid var(--line-strong); color: var(--muted); font: 700 0.7rem var(--mono); }
.section-heading h2 { margin: 0; font-size: 0.8rem; letter-spacing: 0.075em; text-transform: uppercase; }
.field { display: grid; gap: 5px; margin: 0 0 10px; }
.field:last-child { margin-bottom: 0; }
.field > span:first-child { color: var(--ink-soft); font-size: 0.78rem; font-weight: 750; }
.field > span:has(output) { display: flex; justify-content: space-between; gap: 12px; align-items: baseline; }
.field output { color: var(--blue-dark); font: 750 0.84rem var(--mono); }
.field small,
.helper { margin: 0; color: var(--muted); font-size: 0.72rem; line-height: 1.45; }
select,
input[type="number"] {
  width: 100%;
  min-height: 44px;
  border: 1px solid var(--line-strong);
  border-radius: 0;
  background: var(--paper);
  color: var(--ink);
  padding: 7px 10px;
}
input[type="range"] { width: 100%; height: 44px; margin: -7px 0 -9px; accent-color: var(--blue); }
.seed-row { display: grid; grid-template-columns: 1fr 44px; gap: 7px; }
.seed-row button { min-height: 44px; }
.draw-help { margin: 0 0 3px; padding: 10px 11px; border-left: 3px solid var(--rust); background: var(--rust-soft); }
.draw-help p { margin: 0; color: #6b3d29; font-size: 0.75rem; }
.text-button { margin-top: 5px; padding: 3px 0; border: 0; background: transparent; color: var(--blue-dark); font-size: 0.75rem; font-weight: 750; text-decoration: underline; text-underline-offset: 3px; cursor: pointer; }
.preset-row { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 5px; margin: -2px 0 17px; }

button {
  border: 1px solid var(--line-strong);
  border-radius: 0;
  background: var(--paper);
  font-weight: 750;
  cursor: pointer;
  transition: background 160ms ease, border-color 160ms ease, transform 120ms ease;
}
button:hover:not(:disabled) { border-color: var(--blue); background: var(--blue-soft); }
button:active:not(:disabled) { transform: translateY(1px); }
button:disabled { opacity: 0.42; cursor: default; }
.preset-row button { min-height: 44px; padding: 6px 4px; color: var(--blue-dark); font-size: 0.69rem; }
.knot-editor { margin: -5px 0 16px; border-top: 1px solid var(--line); border-bottom: 1px solid var(--line); }
.knot-editor summary { min-height: 44px; display: flex; align-items: center; padding: 8px 0; color: var(--blue-dark); font-size: 0.73rem; font-weight: 750; cursor: pointer; }
.knot-inputs { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 6px; padding: 0 0 9px; }
.knot-inputs label { display: grid; grid-template-columns: auto 1fr; align-items: center; gap: 5px; color: var(--muted); font: 700 0.66rem var(--mono); }
.knot-inputs input { min-width: 0; width: 100%; min-height: 44px; padding: 5px 6px; border: 1px solid var(--line-strong); font: 650 0.68rem var(--mono); }
.action-group { background: #f7f9fa; }
.action-row { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 6px; }
.action-row button { min-height: 44px; padding: 7px 8px; font-size: 0.75rem; }
.primary-button { border-color: var(--teal); background: var(--teal); color: #fff; }
.primary-button:hover:not(:disabled) { border-color: #1f635d; background: #1f635d; }
.primary-button.is-running { border-color: var(--red); background: var(--red); }
.provenance-row { margin-top: 11px; }
.provenance-badge,
.tolerance-badge {
  display: inline-flex;
  align-items: center;
  min-height: 26px;
  padding: 4px 8px;
  border: 1px solid var(--line-strong);
  background: var(--paper);
  font: 750 0.68rem var(--mono);
}
.provenance-badge[data-kind="trained"] { border-color: var(--teal); background: var(--teal-soft); color: #175c56; }
.provenance-badge[data-kind="random"] { border-color: var(--rust); background: var(--rust-soft); color: #743b25; }
.status { min-height: 2.5em; margin: 9px 0 0; color: var(--muted); font-size: 0.72rem; }

.workspace { min-width: 0; padding: clamp(17px, 2vw, 28px); }
.workspace-head,
.panel-head { display: flex; align-items: flex-start; justify-content: space-between; gap: 18px; }
.workspace-head { margin-bottom: 13px; }
.workspace h2 { margin: 0; font-size: clamp(1.35rem, 2vw, 2rem); letter-spacing: -0.03em; }
.workspace h3 { margin: 0; font-size: 1rem; letter-spacing: -0.01em; }
.tolerance-badge { white-space: nowrap; }
.tolerance-badge.is-pass { border-color: var(--teal); background: var(--teal-soft); color: #175c56; }
.tolerance-badge.is-fail { border-color: var(--rust); background: var(--rust-soft); color: #743b25; }

.metric-strip { display: grid; grid-template-columns: repeat(4, minmax(0, 1fr)); margin-bottom: 15px; border: 1px solid var(--line); }
.metric { min-width: 0; padding: 10px 12px; border-right: 1px solid var(--line); background: #f8fafb; }
.metric:last-child { border-right: 0; }
.metric span,
.metric strong,
.metric small { display: block; }
.metric span { color: var(--muted); font-size: 0.68rem; font-weight: 800; letter-spacing: 0.035em; text-transform: uppercase; }
.metric strong { margin-top: 2px; font: 750 1.1rem var(--mono); }
.metric small { margin-top: 1px; color: var(--muted); font-size: 0.67rem; }
.metric-key strong { color: var(--rust); }

.plot-panel,
.model-panel,
.contribution-panel { border: 1px solid var(--line); background: #fbfcfc; }
.plot-panel,
.model-panel { min-width: 0; padding: 15px; }
.main-plot-panel { background: var(--paper); }
.model-formula { max-width: 44ch; margin: 0; color: var(--muted); font: 650 0.72rem/1.45 var(--mono); text-align: right; }
canvas { display: block; width: 100%; background: var(--canvas); }
#mainCanvas { min-height: 310px; height: 310px; margin-top: 7px; border: 1px solid var(--line); touch-action: pan-y; }
#mainCanvas.is-drawable { cursor: crosshair; touch-action: none; }
#mainCanvas.is-knot-editable { cursor: ew-resize; touch-action: none; }
#residualCanvas { min-height: 300px; height: 300px; margin-top: 12px; border: 1px solid var(--line); }
.legend { display: flex; flex-wrap: wrap; align-items: center; gap: 14px; margin-top: 9px; color: var(--muted); font-size: 0.74rem; }
.legend span { display: inline-flex; align-items: center; gap: 6px; }
.line-key { width: 22px; display: inline-block; border-top: 3px solid; }
.target-key { border-color: var(--ink); }
.model-key { border-color: var(--blue); }
.band-key { width: 20px; height: 10px; display: inline-block; border: 1px solid #abc4dc; background: var(--blue-soft); }
.knot-key { width: 9px; height: 9px; display: inline-block; background: var(--rust); transform: rotate(45deg); }
.sample-key { width: 9px; height: 9px; display: inline-block; border: 2px solid var(--blue); border-radius: 50%; background: var(--paper); }
.plot-instruction { margin: 7px 0 0; color: var(--muted); font-size: 0.74rem; }

.analysis-grid { display: grid; grid-template-columns: minmax(0, 1.35fr) minmax(270px, 0.65fr); gap: 14px; margin-top: 14px; }
.compact-value,
.shared-scale { color: var(--rust); font: 750 0.69rem var(--mono); white-space: nowrap; }
.model-panel { border-top: 4px solid var(--ink); }
.model-facts { display: grid; gap: 0; margin: 16px 0 0; }
.model-facts div { display: grid; grid-template-columns: minmax(95px, 0.7fr) minmax(0, 1.3fr); gap: 10px; padding: 8px 0; border-top: 1px solid var(--line); }
.model-facts dt { color: var(--muted); font-size: 0.73rem; font-weight: 750; }
.model-facts dd { margin: 0; font: 650 0.72rem var(--mono); text-align: right; overflow-wrap: anywhere; }
.boundary-note { margin: 15px 0 0; padding: 11px; border-left: 3px solid var(--rust); background: var(--rust-soft); color: #663a27; font-size: 0.76rem; }

.contribution-panel { margin-top: 14px; padding: 16px; }
.contribution-help { margin: 7px 0 0; color: var(--muted); font-size: 0.76rem; }
.contribution-grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(205px, 1fr)); gap: 9px; margin-top: 13px; }
.contribution-card { min-width: 0; padding: 10px; border: 1px solid var(--line); background: var(--paper); }
.contribution-card header { display: flex; justify-content: space-between; gap: 8px; align-items: baseline; }
.contribution-card strong { font-size: 0.78rem; }
.contribution-card header span { color: var(--muted); font: 650 0.68rem var(--mono); }
.contribution-card canvas { height: 145px; margin-top: 7px; border: 1px solid var(--line); }
.contribution-card p { margin: 7px 0 0; color: var(--ink-soft); font: 650 0.71rem/1.45 var(--mono); overflow-wrap: anywhere; }
.contribution-card small { display: block; margin-top: 3px; color: var(--muted); font-size: 0.68rem; }

.classroom-prompts { display: grid; grid-template-columns: minmax(220px, 0.55fr) minmax(0, 1.45fr); gap: 24px; margin-top: 14px; padding: 17px 18px; border-top: 4px solid var(--ink); background: #edf1f3; }
.classroom-prompts ol { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 14px; margin: 0; padding: 0; list-style: none; counter-reset: prompt; }
.classroom-prompts li { position: relative; margin: 0; padding-left: 12px; border-left: 2px solid var(--line-strong); color: var(--muted); font-size: 0.77rem; }
.classroom-prompts strong { color: var(--ink); }

.footer { max-width: 1540px; display: flex; justify-content: space-between; gap: 20px; margin: 0 auto; padding: 0 clamp(18px, 3vw, 44px) 34px; color: var(--muted); font-size: 0.75rem; }
.footer a { font-weight: 750; text-decoration: none; }
.footer a:hover { text-decoration: underline; text-underline-offset: 3px; }

button:focus,
a:focus,
select:focus,
input:focus,
button:focus-visible,
a:focus-visible,
select:focus-visible,
input:focus-visible {
  outline: 3px solid rgba(44, 103, 160, 0.32);
  outline-offset: 2px;
}

@media (max-width: 1120px) {
  .intro { grid-template-columns: minmax(0, 1fr) minmax(310px, 0.72fr); gap: 32px; }
  .lab-shell { grid-template-columns: 276px minmax(0, 1fr); }
  .controls { display: block; }
  .analysis-grid { grid-template-columns: 1fr; }
  .model-panel { border-top-width: 1px; }
  .classroom-prompts { grid-template-columns: 1fr; gap: 12px; }
}

@media (max-width: 820px) {
  .topbar { grid-template-columns: auto 1fr; gap: 10px 16px; }
  .course-mark { grid-column: 2; justify-items: end; }
  .top-links { grid-column: 1 / -1; justify-content: space-between; padding-top: 8px; border-top: 1px solid #4c5962; }
  .intro { grid-template-columns: 1fr; align-items: start; }
  h1 { max-width: 19ch; font-size: clamp(2.2rem, 10vw, 3.8rem); }
  .theorem-card { max-width: 680px; }
  .lane-tabs { width: 100%; }
  .lab-shell { grid-template-columns: 1fr; }
  .controls { position: static; display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .control-group { border-right: 1px solid var(--line); }
  .action-group { grid-column: 1 / -1; }
  .metric-strip { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .metric:nth-child(2) { border-right: 0; }
  .metric:nth-child(-n + 2) { border-bottom: 1px solid var(--line); }
  #mainCanvas { height: 390px; min-height: 390px; }
  .classroom-prompts ol { grid-template-columns: 1fr; gap: 9px; }
}

@media (max-width: 540px) {
  .app { padding-left: 8px; padding-right: 8px; }
  .topbar { padding: 11px 13px; }
  .course-mark span { display: none; }
  .course-mark strong { font-size: 1.05rem; }
  .top-links a { font-size: 0.74rem; }
  .lane-tab { min-height: 55px; padding: 7px 9px; }
  .lane-index, .lane-tab small { display: none; }
  .controls { grid-template-columns: 1fr; }
  .control-group, .action-group { grid-column: auto; border-right: 0; }
  .workspace { padding: 12px; }
  .workspace-head, .panel-head { display: grid; }
  .tolerance-badge { justify-self: start; }
  .model-formula { text-align: left; }
  #mainCanvas { height: 360px; min-height: 360px; }
  #residualCanvas { height: 270px; min-height: 270px; }
  .contribution-panel { padding: 11px; }
  .contribution-grid { grid-template-columns: 1fr; }
  .shared-scale { white-space: normal; }
  .footer { flex-direction: column; gap: 7px; }
}

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  *, *::before, *::after { transition-duration: 0.01ms !important; }
}