:root {
  color-scheme: light;
  --ink: #172b3b;
  --ink-soft: #41576a;
  --muted: #718394;
  --paper: #fbfcfd;
  --surface: #ffffff;
  --surface-blue: #eef4f7;
  --surface-quiet: #f4f7f8;
  --line: #ced8de;
  --line-strong: #9bacb7;
  --blue: #315c78;
  --blue-deep: #204660;
  --bridge: #ce683e;
  --bridge-soft: #fff0e8;
  --gold: #ddae52;
  --good: #23684d;
  --warning: #9a492d;
  --shadow: 0 16px 44px rgba(25, 47, 63, .08);
  --font-body: "Microsoft YaHei", "Noto Sans CJK SC", "PingFang SC", sans-serif;
  --font-display: "STSong", "SimSun", "Source Han Serif SC", serif;
  --font-math: "Cambria Math", "STIX Two Math", "Times New Roman", serif;
  font-family: var(--font-body);
  color: var(--ink);
  background: var(--paper);
}
* { box-sizing: border-box; }
html { min-width: 320px; background: var(--paper); }
body {
  margin: 0;
  min-height: 100vh;
  background:
    linear-gradient(90deg, rgba(49,92,120,.025) 1px, transparent 1px) 0 0 / 44px 44px,
    var(--paper);
  color: var(--ink);
}
button, textarea, select { font: inherit; }
button, select { color: inherit; }
button { cursor: pointer; }
button:disabled { cursor: not-allowed; opacity: .5; }
:focus-visible { outline: 3px solid var(--gold); outline-offset: 3px; }
a { color: inherit; }
.masthead {
  min-height: 76px;
  padding: 14px clamp(18px, 5vw, 76px);
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 18px;
  border-bottom: 1px solid var(--line);
  background: rgba(251,252,253,.96);
}
.brand { display: inline-flex; align-items: center; gap: 13px; text-decoration: none; }
.brand-mark {
  display: grid;
  place-items: center;
  width: 42px;
  height: 42px;
  border: 1px solid var(--blue);
  color: var(--blue);
  font-family: var(--font-display);
  font-size: 26px;
  transform: rotate(0deg);
}
.brand-kicker, .brand-name { display: block; }
.brand-kicker { color: var(--muted); font: 700 9px/1.4 var(--font-body); letter-spacing: .18em; }
.brand-name { margin-top: 3px; font: 700 18px/1.2 var(--font-display); letter-spacing: .08em; }
.status-pills { display: flex; flex-wrap: wrap; justify-content: flex-end; gap: 8px; }
.pill {
  border: 1px solid var(--line);
  border-radius: 999px;
  padding: 6px 10px;
  font-size: 11px;
  line-height: 1;
  letter-spacing: .04em;
  white-space: nowrap;
}
.pill-local { color: var(--blue-deep); background: var(--surface-blue); }
.pill-review { color: #7a4b26; background: #fbf5e8; border-color: #ead7aa; }
.status-dot { display: inline-block; width: 6px; height: 6px; margin: 0 6px 1px 0; border-radius: 50%; background: var(--blue); }
.app-shell { min-height: calc(100vh - 130px); padding: 28px clamp(16px, 5vw, 78px) 50px; }
.loading-message { color: var(--muted); text-align: center; padding: 120px 0; }
.site-footer {
  display: flex;
  justify-content: space-between;
  gap: 16px;
  border-top: 1px solid var(--line);
  padding: 14px clamp(18px,5vw,76px);
  color: var(--muted);
  background: var(--paper);
  font-size: 11px;
}
.eyebrow { margin: 0 0 11px; color: var(--blue); font: 700 10px/1.4 var(--font-body); letter-spacing: .16em; text-transform: uppercase; }
.page-wrap { width: min(1220px, 100%); margin: 0 auto; }
.hero {
  display: grid;
  grid-template-columns: minmax(0, 1.04fr) minmax(300px, .96fr);
  gap: clamp(24px, 5vw, 66px);
  align-items: center;
  padding: clamp(22px, 5vw, 58px) 0 42px;
}
.hero-copy { max-width: 620px; }
.hero h1 { margin: 0; max-width: 680px; font: 500 clamp(36px, 5vw, 66px)/1.18 var(--font-display); letter-spacing: .025em; }
.hero h1 em { color: var(--bridge); font-style: normal; }
.hero-lead { max-width: 520px; margin: 22px 0 0; color: var(--ink-soft); font-size: clamp(15px,1.5vw,18px); line-height: 1.9; }
.hero-note { margin: 17px 0 0; color: var(--muted); font-size: 12px; line-height: 1.7; }
.hero-art {
  position: relative;
  min-height: 335px;
  padding: 22px;
  border: 1px solid var(--line);
  background: var(--surface);
  box-shadow: var(--shadow);
}
.hero-art::before { content: "T00 · 题目示意"; position: absolute; top: 13px; left: 18px; color: var(--muted); font-size: 10px; letter-spacing: .12em; }
.hero-art img { display: block; width: 100%; height: 285px; object-fit: contain; margin: 15px auto 0; }
.entry-actions { display: flex; flex-wrap: wrap; gap: 11px; margin-top: 27px; }
.button {
  min-height: 44px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 9px;
  border: 1px solid var(--blue);
  border-radius: 2px;
  padding: 11px 16px;
  background: var(--blue);
  color: white;
  font-weight: 650;
  font-size: 14px;
  text-decoration: none;
  transition: background-color .16s ease, transform .16s ease;
}
.button:hover { background: var(--blue-deep); transform: translateY(-1px); }
.button.secondary { color: var(--blue-deep); background: white; }
.button.secondary:hover { background: var(--surface-blue); }
.button.quiet { color: var(--ink-soft); background: transparent; border-color: var(--line-strong); }
.button.quiet:hover { background: var(--surface-quiet); }
.button.danger { color: var(--warning); border-color: #d7aa98; background: white; }
.button.small { min-height: 36px; padding: 8px 11px; font-size: 12px; }
.entry-bottom {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 16px;
  border-top: 1px solid var(--line);
  padding: 21px 0 0;
}
.info-note { padding: 15px 17px; border-left: 2px solid var(--blue); background: var(--surface-blue); color: var(--ink-soft); font-size: 13px; line-height: 1.75; }
.info-note strong { display: block; margin-bottom: 4px; color: var(--ink); font-size: 12px; letter-spacing: .03em; }
.demo-entry { display: flex; align-items: center; justify-content: space-between; gap: 16px; padding: 15px 17px; border: 1px dashed var(--line-strong); color: var(--ink-soft); font-size: 12px; line-height: 1.6; }
.demo-entry strong { color: var(--ink); }
.section-heading { margin: 0 0 18px; font: 600 22px/1.3 var(--font-display); }
.mini-label { color: var(--muted); font-size: 10px; letter-spacing: .1em; }
.storage-banner { margin: 0 auto 17px; width: min(1220px, 100%); padding: 11px 14px; border-left: 3px solid var(--good); background: #eaf5ef; color: #245b45; font-size: 12px; }
.storage-banner.error { border-color: var(--warning); background: #fff1eb; color: #833f27; }
.session-topline { width: min(1220px,100%); margin: 0 auto 15px; display: flex; align-items: center; justify-content: space-between; gap: 12px; color: var(--muted); font-size: 11px; }
.session-meta { display: flex; flex-wrap: wrap; align-items: center; gap: 8px; }
.session-code { color: var(--blue-deep); font-weight: 700; letter-spacing: .08em; }
.learner-layout { width: min(1220px,100%); margin: 0 auto; display: grid; grid-template-columns: minmax(0, 1.15fr) minmax(315px, .85fr); gap: 22px; align-items: start; }
.question-sheet, .work-panel, .teacher-panel, .record-panel, .completion-panel {
  border: 1px solid var(--line);
  background: var(--surface);
  box-shadow: 0 7px 24px rgba(25,47,63,.045);
}
.question-sheet { overflow: hidden; }
.question-header { display:flex; justify-content:space-between; align-items:center; padding: 17px 20px; border-bottom: 1px solid var(--line); background: #f7fafb; }
.question-index { display:flex; align-items:baseline; gap:10px; }
.question-index strong { color:var(--blue); font:600 25px/1 var(--font-display); letter-spacing:.05em; }
.question-index span { color:var(--muted); font-size:11px; }
.question-prompt { padding: 24px clamp(18px, 3vw, 34px) 16px; font: 500 clamp(18px, 2vw, 24px)/1.8 var(--font-display); letter-spacing: .01em; }
.figure-frame { margin: 0 clamp(16px, 2.5vw, 30px) 26px; padding: 12px; border: 1px solid #e0e7eb; background: #fcfdfe; }
.figure-frame img { display:block; width:100%; max-height:360px; object-fit:contain; }
.figure-caption { margin: 8px 0 0; color:var(--muted); text-align:center; font-size:10px; line-height:1.5; }
.pencil-note { margin:0 clamp(18px,3vw,34px) 20px; color:var(--muted); font-size:12px; line-height:1.6; }
.work-panel { position: sticky; top: 16px; }
.panel-head { display:flex; justify-content:space-between; align-items:flex-start; gap:12px; padding:18px 20px 13px; border-bottom:1px solid var(--line); }
.panel-head h2 { margin:0; font:600 20px/1.3 var(--font-display); }
.panel-head p { margin:6px 0 0; color:var(--muted); font-size:11px; line-height:1.55; }
.panel-body { padding: 17px 20px 20px; }
.text-label { display:block; margin:0 0 8px; color:var(--ink-soft); font-size:12px; font-weight:700; }
textarea {
  display:block;
  width:100%;
  min-height:145px;
  resize:vertical;
  padding:12px 13px;
  border:1px solid var(--line-strong);
  border-radius:2px;
  background:#fff;
  color:var(--ink);
  font-family:var(--font-math), var(--font-body);
  font-size:15px;
  line-height:1.75;
}
textarea::placeholder { color:#9aa8b1; font-family:var(--font-body); font-size:12px; }
.field-hint { margin:7px 0 0; color:var(--muted); font-size:10px; line-height:1.55; }
.save-row { display:flex; justify-content:space-between; align-items:center; gap:10px; margin-top:11px; }
.save-state { color:var(--muted); font-size:11px; line-height:1.5; }
.save-state.success { color:var(--good); }
.save-state.error { color:var(--warning); }
.self-report-row { display:flex; flex-wrap:wrap; gap:8px; margin:17px 0 0; }
.self-report-row .button { flex:1; }
.help-actions { display:grid; grid-template-columns:1fr 1fr; gap:8px; margin-top:17px; padding-top:16px; border-top:1px solid var(--line); }
.help-actions .button { min-height:42px; padding:9px 10px; font-size:12px; }
.end-row { display:flex; justify-content:space-between; gap:8px; margin-top:13px; }
.end-row .button { flex:1; }
.paper-option { margin-top:12px; padding:10px 12px; background:var(--surface-quiet); color:var(--muted); font-size:11px; line-height:1.6; }
.help-card { width:min(1220px,100%); margin:18px auto 0; padding:0; border:1px solid #d9c48f; background:#fffdf7; box-shadow:var(--shadow); }
.help-card-head { display:flex; justify-content:space-between; gap:14px; padding:16px 20px; border-bottom:1px solid #eadfbe; background:#fbf7e9; }
.help-card-head h2 { margin:0; font:600 21px/1.3 var(--font-display); }
.help-card-head p { margin:5px 0 0; color:var(--muted); font-size:11px; }
.help-card-body { padding:19px clamp(18px,4vw,42px) 26px; line-height:1.85; }
.help-block { display:grid; grid-template-columns:170px 1fr; gap:15px; padding:14px 0; border-bottom:1px solid #ece5d1; }
.help-block:last-child { border-bottom:0; }
.help-block h3 { margin:0; color:var(--blue-deep); font-size:13px; }
.help-block p, .proof-line { margin:0; font-size:15px; }
.proof-list { margin:0; padding-left:22px; }
.proof-list li { padding:4px 0 4px 5px; font-size:15px; }
.bridge-diagram { display:grid; grid-template-columns:1fr auto 1fr auto 1fr auto 1fr; align-items:stretch; gap:8px; margin:18px 0 8px; }
.bridge-step { min-height:64px; display:flex; flex-direction:column; justify-content:center; padding:10px; border:1px solid var(--line); background:white; text-align:center; }
.bridge-step small { display:block; margin-bottom:5px; color:var(--muted); font-size:9px; letter-spacing:.08em; }
.bridge-step strong { font:600 15px/1.3 var(--font-math), var(--font-body); }
.bridge-step.connector { border-color:#e8b397; background:var(--bridge-soft); }
.bridge-step.connector strong { color:var(--bridge); }
.bridge-arrow { align-self:center; color:var(--bridge); font-size:16px; }
.teacher-caution { margin:17px 0 0; padding:12px 14px; border-left:2px solid var(--bridge); background:var(--bridge-soft); color:#74442f; font-size:12px; line-height:1.7; }
.support-picker { display:grid; grid-template-columns:1fr 1fr; gap:8px; margin-top:12px; }
.support-picker .button { min-height:42px; font-size:12px; }
.task-picker { width:min(1220px,100%); margin:15px auto 0; padding:15px 18px; border:1px solid var(--line); background:var(--surface); }
.task-picker h3 { margin:0 0 11px; font:600 15px/1.3 var(--font-display); }
.task-list { display:grid; grid-template-columns:repeat(8,minmax(48px,1fr)); gap:7px; }
.task-chip { min-height:36px; border:1px solid var(--line); background:white; color:var(--ink-soft); font:700 11px/1 var(--font-body); letter-spacing:.06em; }
.task-chip:hover { border-color:var(--blue); background:var(--surface-blue); }
.task-chip.current { border-color:var(--blue); background:var(--blue); color:white; }
.feedback { margin:12px 0 0; padding:10px 12px; border-left:3px solid var(--blue); background:var(--surface-blue); color:var(--ink-soft); font-size:12px; line-height:1.6; }
.feedback.error { border-color:var(--warning); background:#fff1eb; color:#813f28; }
.page-title-row { display:flex; justify-content:space-between; align-items:flex-end; gap:18px; margin:5px 0 21px; }
.page-title-row h1 { margin:0; font:500 clamp(31px,4vw,48px)/1.2 var(--font-display); }
.page-title-row p { margin:8px 0 0; max-width:700px; color:var(--ink-soft); font-size:13px; line-height:1.7; }
.teacher-panel, .record-panel, .completion-panel { width:min(1100px,100%); margin:0 auto; }
.teacher-panel { padding:24px clamp(18px,4vw,38px) 30px; }
.teacher-banner { display:flex; align-items:center; justify-content:space-between; gap:15px; padding:13px 16px; border:1px dashed #b98e58; background:#fff9ee; color:#754c20; font-size:12px; line-height:1.6; }
.teacher-banner strong { display:block; margin-bottom:2px; }
.teacher-grid { display:grid; grid-template-columns:1fr 1fr; gap:24px; margin-top:22px; }
.form-section { padding:16px; border:1px solid var(--line); background:#fff; }
.form-section h2 { margin:0 0 6px; font:600 18px/1.3 var(--font-display); }
.form-section > p { margin:0 0 13px; color:var(--muted); font-size:11px; line-height:1.55; }
.choice-list { display:grid; gap:7px; }
.choice-row { display:flex; align-items:flex-start; gap:9px; padding:9px 10px; border:1px solid #e4e9ec; background:#fcfdfd; color:var(--ink-soft); font-size:12px; line-height:1.55; }
.choice-row input { flex:0 0 auto; margin-top:3px; accent-color:var(--blue); }
.status-list { display:grid; gap:10px; }
.status-row { display:grid; grid-template-columns:minmax(130px,1fr) 185px; align-items:center; gap:10px; padding:9px 0; border-bottom:1px solid #edf0f1; color:var(--ink-soft); font-size:12px; line-height:1.5; }
.status-row:last-child { border-bottom:0; }
select {
  min-height:36px;
  border:1px solid var(--line-strong);
  border-radius:2px;
  padding:6px 8px;
  background:white;
  color:var(--ink);
  font-size:11px;
}
.demo-action-row { display:flex; flex-wrap:wrap; gap:9px; margin-top:19px; }
.demo-results { margin-top:20px; }
.result-banner { padding:13px 15px; border-left:3px solid var(--blue); background:var(--surface-blue); color:var(--ink-soft); font-size:12px; line-height:1.7; }
.result-banner.conflict { border-color:var(--warning); background:#fff1eb; color:#813f28; }
.candidate-grid { display:grid; grid-template-columns:repeat(auto-fit,minmax(230px,1fr)); gap:11px; margin-top:12px; }
.candidate-card { padding:14px; border:1px solid var(--line); background:#fff; }
.candidate-card h3 { display:flex; align-items:center; gap:9px; margin:0 0 8px; font:600 18px/1.3 var(--font-display); }
.candidate-card .candidate-mode { display:inline-flex; padding:4px 7px; border-radius:999px; background:var(--surface-blue); color:var(--blue-deep); font:700 9px/1 var(--font-body); }
.candidate-card.teacher-supported { border-color:#e4c28c; background:#fffdf7; }
.candidate-card.teacher-supported .candidate-mode { background:#fbf1dd; color:#795924; }
.candidate-card p { margin:7px 0 0; color:var(--ink-soft); font-size:11px; line-height:1.65; }
.blocked-card { margin-top:10px; padding:12px 14px; border:1px dashed var(--line-strong); color:var(--muted); font-size:11px; line-height:1.7; }
.record-actions { display:flex; flex-wrap:wrap; gap:9px; margin:17px 0; }
.record-list { display:grid; gap:11px; margin-top:15px; }
.record-item { padding:15px 16px; border:1px solid var(--line); background:#fff; }
.record-item h3 { margin:0 0 6px; font:600 16px/1.3 var(--font-display); }
.record-item p { margin:3px 0; color:var(--muted); font-size:11px; line-height:1.55; }
.record-item .record-actions { margin:11px 0 0; }
.completion-panel { padding:clamp(22px,4vw,42px); text-align:center; }
.completion-panel .finish-mark { display:inline-grid; place-items:center; width:52px; height:52px; border:1px solid var(--blue); color:var(--blue); font:24px/1 var(--font-display); }
.completion-panel h1 { margin:17px 0 8px; font:500 38px/1.2 var(--font-display); }
.completion-panel p { max-width:680px; margin:0 auto; color:var(--ink-soft); font-size:14px; line-height:1.8; }
.completion-actions { display:flex; flex-wrap:wrap; justify-content:center; gap:9px; margin-top:21px; }
.empty-state { padding:30px 20px; border:1px dashed var(--line-strong); color:var(--muted); text-align:center; font-size:12px; line-height:1.7; }
.alert-line { margin:12px 0 0; color:var(--warning); font-size:12px; line-height:1.6; }
.demo-footer-note { margin-top:17px; color:var(--muted); font-size:10px; line-height:1.65; }
.lesson-shell { width:min(1220px,100%); margin:20px auto 0; border:1px solid var(--line-strong); background:#fff; box-shadow:var(--shadow); scroll-margin-top:14px; }
.lesson-head { display:flex; justify-content:space-between; align-items:start; gap:22px; padding:21px 25px 18px; border-bottom:1px solid var(--line); }
.lesson-head .eyebrow { margin:0 0 5px; }
.lesson-head h2 { margin:0; font:600 clamp(20px,2vw,27px)/1.35 var(--font-display); }
.lesson-head p:last-child { margin:6px 0 0; color:var(--muted); font-size:12px; }
.lesson-head-actions { display:flex; flex-wrap:wrap; justify-content:flex-end; gap:8px; }
.lesson-steps { display:grid; grid-template-columns:repeat(6,minmax(0,1fr)); gap:0; border-bottom:1px solid var(--line); background:var(--surface-quiet); }
.lesson-step { display:flex; align-items:center; gap:7px; min-height:53px; padding:8px 10px; border:0; border-right:1px solid var(--line); background:transparent; text-align:left; color:var(--ink-soft); }
.lesson-step:last-child { border-right:0; }
.lesson-step:hover { background:var(--surface-blue); }
.lesson-step.active { background:var(--blue-deep); color:white; }
.lesson-step span { font:700 10px/1 var(--font-math); opacity:.7; }
.lesson-step strong { font-size:12px; font-weight:600; white-space:nowrap; }
.lesson-columns { display:grid; grid-template-columns:minmax(210px,.82fr) minmax(310px,1.35fr) minmax(290px,1fr); min-height:445px; }
.lesson-original, .lesson-figure, .lesson-reasoning { min-width:0; margin:0; padding:22px 20px; }
.lesson-original, .lesson-figure { border-right:1px solid var(--line); }
.lesson-column-label { margin-bottom:13px; color:var(--blue); font-size:10px; font-weight:700; letter-spacing:.08em; }
.lesson-original > p:first-of-type { margin:0; font:500 16px/1.9 var(--font-display); }
.lesson-target { margin-top:24px; padding:12px 13px; border-left:3px solid var(--bridge); background:var(--bridge-soft); font-size:12px; }
.lesson-target strong { display:block; margin-top:3px; font:700 20px/1.4 var(--font-math); }
.lesson-original .lesson-note { margin-top:18px; color:var(--muted); font-size:11px; line-height:1.7; }
.lesson-figure { display:flex; flex-direction:column; background:#fcfdfe; }
.lesson-figure-image { position:relative; width:100%; aspect-ratio:1000/540; margin:auto 0; }
.lesson-figure-image img, .diagram-overlay { position:absolute; inset:0; display:block; width:100%; height:100%; }
.lesson-figure-image img { object-fit:contain; }
.diagram-overlay { overflow:visible; pointer-events:none; }
.diagram-focus { fill:none; stroke:var(--bridge); stroke-width:9; stroke-linecap:round; stroke-linejoin:round; opacity:.82; }
.diagram-focus.outline { stroke-width:7; opacity:.6; }
.diagram-focus.pending { stroke-dasharray:16 12; opacity:.82; }
.diagram-focus.whole { stroke:var(--blue); stroke-width:5; opacity:.82; }
.diagram-focus.removed { stroke:var(--bridge); stroke-width:9; opacity:.9; }
.diagram-focus.remaining { stroke:var(--good); stroke-width:9; opacity:.9; }
.diagram-focus.proof-object { stroke:var(--blue); stroke-width:4; opacity:.56; }
.diagram-focus.proof-target { stroke:var(--bridge); stroke-width:6; opacity:.78; }
.lesson-phase-map { display:grid; gap:5px; margin:10px 0 12px; }
.lesson-phase { display:grid; grid-template-columns:66px minmax(0,1fr); gap:9px; align-items:center; padding:7px 9px; border-left:3px solid var(--blue); background:var(--surface-blue); }
.lesson-phase span { color:var(--ink-soft); font-size:10px; }
.lesson-phase strong { font:600 11px/1.5 var(--font-math),var(--font-body); overflow-wrap:anywhere; }
.lesson-phase.phase-3 { border-color:var(--bridge); background:var(--bridge-soft); }
.lesson-phase.phase-4 { border-color:var(--good); background:#eaf4ef; }
.lesson-figure figcaption { margin-top:auto; color:var(--muted); font-size:10px; line-height:1.5; text-align:center; }
.lesson-reasoning { display:flex; flex-direction:column; gap:17px; }
.lesson-reasoning section + section { border-top:1px solid var(--line); padding-top:15px; }
.lesson-reasoning h3 { margin:0 0 9px; font:600 18px/1.4 var(--font-display); }
.lesson-reasoning p { margin:0; font-size:12px; line-height:1.75; }
.lesson-screen-text { margin:13px 0; padding:11px 12px; background:var(--surface-blue); color:var(--blue-deep); font:700 14px/1.6 var(--font-math),var(--font-body); }
.lesson-reasoning .lesson-narration { color:var(--ink-soft); font-size:12px; line-height:1.8; }
.lesson-core-label { margin-top:16px; margin-bottom:5px; }
.lesson-jump-context { margin:12px 0 0; padding:10px 12px; border-left:2px solid var(--bridge); background:var(--bridge-soft); color:var(--ink-soft); font-size:11px; line-height:1.7; }
.lesson-jump-context .button { margin-top:7px; }
.lesson-results ul { margin:0; padding-left:17px; }
.lesson-results li { margin:5px 0; font:600 12px/1.6 var(--font-math),var(--font-body); }
.lesson-results p, .lesson-route p { color:var(--ink-soft); }
.lesson-footer { display:flex; justify-content:space-between; align-items:center; gap:16px; padding:13px 20px; border-top:1px solid var(--line); background:var(--surface-quiet); }
.lesson-progress { color:var(--muted); font-size:11px; }
.lesson-move, .lesson-next-actions { display:flex; flex-wrap:wrap; gap:8px; }
.lesson-next { padding:20px 24px 24px; border-top:1px solid var(--line); }
.lesson-next h3 { margin:0 0 12px; font:600 17px/1.4 var(--font-display); }
.lesson-next p { margin:11px 0 0; color:var(--muted); font-size:11px; line-height:1.6; }
.hidden { display:none !important; }
@media (max-width: 1050px) {
  .lesson-columns { grid-template-columns:minmax(180px,.75fr) minmax(280px,1.25fr); }
  .lesson-reasoning { grid-column:1 / -1; display:grid; grid-template-columns:1.35fr 1fr 1fr; border-top:1px solid var(--line); }
  .lesson-reasoning section + section { border-top:0; border-left:1px solid var(--line); padding-top:0; padding-left:16px; }
}
@media (max-width: 860px) {
  .hero { grid-template-columns:1fr; }
  .hero-art { min-height:280px; }
  .hero-art img { height:225px; }
  .learner-layout { grid-template-columns:1fr; }
  .work-panel { position:static; }
  .teacher-grid { grid-template-columns:1fr; }
  .work-panel { order:2; }
  .lesson-head { flex-direction:column; }
  .lesson-head-actions { justify-content:flex-start; }
  .lesson-steps { grid-template-columns:repeat(3,1fr); }
  .lesson-step:nth-child(3) { border-right:0; }
  .lesson-step:nth-child(-n+3) { border-bottom:1px solid var(--line); }
}
@media (max-width: 560px) {
  .masthead { align-items:flex-start; padding:12px 14px; }
  .brand-mark { width:36px; height:36px; }
  .brand-kicker { font-size:8px; }
  .brand-name { font-size:16px; }
  .status-pills { max-width:150px; gap:5px; }
  .pill { padding:5px 7px; font-size:9px; }
  .app-shell { padding:18px 13px 36px; }
  .hero { padding:19px 0 28px; gap:20px; }
  .hero h1 { font-size:38px; }
  .hero-art { min-height:235px; padding:14px; }
  .hero-art img { height:185px; }
  .entry-bottom { grid-template-columns:1fr; }
  .site-footer { flex-direction:column; padding:12px 14px; font-size:10px; }
  .question-header { padding:14px; }
  .question-prompt { padding:18px 16px 12px; font-size:19px; }
  .figure-frame { margin:0 12px 18px; padding:7px; }
  .panel-head, .panel-body { padding-left:15px; padding-right:15px; }
  .help-block { grid-template-columns:1fr; gap:4px; }
  .bridge-diagram { grid-template-columns:1fr; gap:5px; }
  .bridge-arrow { transform:rotate(90deg); justify-self:center; line-height:1; }
  .support-picker { grid-template-columns:1fr; }
  .task-list { grid-template-columns:repeat(4,1fr); }
  .session-topline { align-items:flex-start; flex-direction:column; }
  .status-row { grid-template-columns:1fr; gap:6px; }
  .status-row select { width:100%; }
  .entry-actions .button { flex:1 1 100%; }
  .page-title-row { align-items:flex-start; flex-direction:column; }
  .lesson-head { padding:16px; }
  .lesson-columns { display:block; }
  .lesson-original, .lesson-figure, .lesson-reasoning { border-right:0; border-bottom:1px solid var(--line); padding:16px; }
  .lesson-original > p:first-of-type { font-size:14px; }
  .lesson-figure-image { margin:0 auto; }
  .lesson-reasoning { display:block; }
  .lesson-reasoning section + section { margin-top:15px; padding:15px 0 0; border-left:0; border-top:1px solid var(--line); }
  .lesson-footer { align-items:flex-start; flex-direction:column; }
  .lesson-next { padding:16px; }
  .lesson-next-actions .button { flex:1 1 100%; }
}
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { scroll-behavior:auto !important; transition:none !important; animation:none !important; }
}
.video-stage{padding:16px 24px 0;background:#e9eef0}
.video-stage video{display:block;width:100%;max-height:72vh;aspect-ratio:16/9;object-fit:contain;background:#eef2f5;border-radius:8px}
.video-status,.video-source-note{margin:0;padding:14px 24px;font-size:13px;line-height:1.75;color:var(--muted,#5b6871)}
.video-source-note{border-top:1px solid #dce3e3;background:#f8faf9}
.video-lesson .lesson-next{margin-top:0}
@media(max-width:640px){.video-stage{padding:10px 8px}.video-status,.video-source-note{padding:12px 14px}.video-lesson .lesson-head-actions{flex-wrap:wrap}}