:root {
  --paper: #f4efe3;
  --paper-2: #efe8d7;
  --card: #fbf7ec;
  --ink: #191710;
  --ink-2: #4f4a3c;
  --ink-3: #7a7463;
  --rule: rgba(25, 23, 16, 0.2);
  --rule-strong: rgba(25, 23, 16, 0.55);
  --accent: #c04020;
  --accent-soft: rgba(192, 64, 32, 0.09);
  --ok: #3f7a4f;
  --warn: #b07a12;
  --bad: #b3352c;
  --r: 2px;
  --sans: "Helvetica Neue", "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei", Arial, sans-serif;
  --serif: Georgia, "Times New Roman", "Songti SC", SimSun, "Noto Serif SC", serif;
  --mono: "SF Mono", "Cascadia Mono", Consolas, "Courier New", monospace;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  min-height: 100vh;
  color: var(--ink);
  font-family: var(--sans);
  font-size: 16px;
  line-height: 1.65;
  background-color: var(--paper);
  background-image: radial-gradient(rgba(25, 23, 16, 0.055) 0.7px, transparent 0.7px);
  background-size: 18px 18px;
  -webkit-font-smoothing: antialiased;
}

::selection { background: rgba(192, 64, 32, 0.2); }

a { color: inherit; text-decoration: none; }

/* ---------- layout shells ---------- */

.site-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  max-width: 1120px;
  margin: 0 auto;
  padding: 22px 24px 16px;
  border-bottom: 1px solid var(--rule-strong);
}

.brand {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  font-family: var(--serif);
  font-weight: 700;
  font-size: 17px;
  letter-spacing: 0.01em;
}

.brand .logo {
  width: 13px;
  height: 13px;
  background: var(--accent);
  border-radius: 1px;
  transform: rotate(45deg);
  flex: 0 0 auto;
}

.header-side {
  display: flex;
  align-items: center;
  gap: 8px;
}

.lang-toggle {
  font-family: var(--sans);
  font-size: 11px;
  font-weight: 600;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--ink);
  background: transparent;
  border: 1px solid var(--rule-strong);
  border-radius: var(--r);
  padding: 7px 12px;
  cursor: pointer;
  transition: background 0.15s, color 0.15s, border-color 0.15s;
}

.lang-toggle:hover {
  background: var(--ink);
  color: var(--paper);
  border-color: var(--ink);
}

main {
  max-width: 1120px;
  margin: 0 auto;
  padding: 8px 24px 64px;
}

/* ---------- landing ---------- */

.hero {
  padding: 64px 0 44px;
  border-bottom: 1px solid var(--rule);
}

.kicker {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  margin: 0 0 18px;
  font-family: var(--mono);
  font-size: 11px;
  letter-spacing: 0.18em;
  text-transform: uppercase;
  color: var(--accent);
}

.kicker::before {
  content: "";
  width: 7px;
  height: 7px;
  background: var(--accent);
}

.hero h1 {
  margin: 0 0 18px;
  font-family: var(--serif);
  font-weight: 500;
  font-size: clamp(2.4rem, 6.2vw, 4.1rem);
  line-height: 1.06;
  letter-spacing: -0.025em;
  color: var(--ink);
}

.hero-sub {
  margin: 0;
  max-width: 56ch;
  color: var(--ink-2);
  font-size: clamp(1rem, 1.6vw, 1.1rem);
}

.section-title {
  margin: 34px 0 18px;
  padding-top: 14px;
  border-top: 1px solid var(--rule-strong);
  font-family: var(--mono);
  font-size: 11px;
  font-weight: 600;
  letter-spacing: 0.2em;
  text-transform: uppercase;
  color: var(--ink-3);
}

/* ---------- topic cards ---------- */

.topics {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
  gap: 18px;
}

.card {
  position: relative;
  border: 1px solid var(--rule-strong);
  border-radius: var(--r);
  background: var(--card);
  overflow: hidden;
}

.topic-card {
  display: flex;
  flex-direction: column;
  min-height: 306px;
  transition: transform 0.18s ease, box-shadow 0.18s ease, border-color 0.18s ease;
}

.topic-card.live:hover {
  transform: translate(-2px, -2px);
  border-color: var(--ink);
  box-shadow: 6px 6px 0 var(--accent);
}

.topic-card.soon { opacity: 0.7; }

.thumb {
  height: 130px;
  display: grid;
  place-items: center;
  background: var(--paper-2);
  border-bottom: 1px solid var(--rule);
}

.thumb svg { width: 150px; height: 108px; overflow: visible; }

.topic-body {
  padding: 16px 18px 20px;
  display: flex;
  flex-direction: column;
  gap: 8px;
  flex: 1;
}

.topic-body h2 {
  margin: 0;
  font-family: var(--serif);
  font-weight: 600;
  font-size: 18px;
  line-height: 1.35;
}

.topic-body p { margin: 0; color: var(--ink-2); font-size: 14px; }

.topic-body .go {
  margin-top: auto;
  padding-top: 12px;
  font-family: var(--mono);
  font-size: 11px;
  font-weight: 600;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--accent);
}

.badge {
  align-self: flex-start;
  padding: 3px 8px;
  font-family: var(--mono);
  font-size: 10px;
  font-weight: 600;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  border: 1px solid currentColor;
  border-radius: var(--r);
}

.badge.live { color: var(--accent); }

.badge.soon { color: var(--ink-3); }

/* ---------- lab ---------- */

.lab {
  display: grid;
  grid-template-columns: minmax(0, 1fr) 370px;
  gap: 22px;
  align-items: start;
  margin-top: 26px;
}

.stage-figure { margin: 0; }

.stage-card {
  position: relative;
  border: 1px solid var(--ink);
  border-radius: var(--r);
  background: radial-gradient(120% 110% at 50% 0%, #0b111e 0%, #04060c 72%);
  overflow: hidden;
  min-height: 420px;
  aspect-ratio: 1.02;
  box-shadow: 8px 8px 0 rgba(25, 23, 16, 0.08);
}

#sky { display: block; width: 100%; height: 100%; }

.hud {
  position: absolute;
  inset: 14px 14px auto;
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 10px;
  pointer-events: none;
}

.hud-badge {
  display: inline-flex;
  align-items: center;
  gap: 7px;
  padding: 5px 10px;
  font-family: var(--mono);
  font-size: 10px;
  font-weight: 600;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: #f4efe3;
  background: rgba(13, 16, 10, 0.55);
  border: 1px solid rgba(244, 239, 227, 0.35);
  border-radius: var(--r);
}

.hud-badge::before {
  content: "";
  width: 7px;
  height: 7px;
  border-radius: 50%;
  background: #f4efe3;
}

.hud-badge.s-circle::before { background: #8fc79c; }
.hud-badge.s-ellipse::before { background: #b4a6ea; }
.hud-badge.s-escape::before { background: #e2a33e; }
.hud-badge.s-crash::before { background: #e0705f; }

.hud-speed {
  padding: 5px 10px;
  font-family: var(--mono);
  font-size: 10px;
  letter-spacing: 0.06em;
  color: #f4efe3;
  background: rgba(13, 16, 10, 0.55);
  border: 1px solid rgba(244, 239, 227, 0.35);
  border-radius: var(--r);
}

.stage-figure figcaption {
  margin-top: 10px;
  font-family: var(--mono);
  font-size: 11px;
  line-height: 1.6;
  letter-spacing: 0.02em;
  color: var(--ink-3);
}

.panel {
  border: 1px solid var(--rule-strong);
  border-radius: var(--r);
  background: var(--card);
  padding: 24px;
  position: sticky;
  top: 16px;
}

.panel h1 {
  margin: 0 0 8px;
  font-family: var(--serif);
  font-weight: 600;
  font-size: 26px;
  letter-spacing: -0.01em;
}

.lede { margin: 0 0 22px; color: var(--ink-2); font-size: 14px; }

.control { margin-bottom: 16px; }

.control-head {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 10px;
  margin-bottom: 8px;
}

.control-head label {
  font-family: var(--mono);
  font-size: 10px;
  font-weight: 600;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--ink-3);
}

.control-head output {
  font-family: var(--mono);
  font-size: 12px;
  color: var(--accent);
}

input[type="range"] {
  -webkit-appearance: none;
  appearance: none;
  width: 100%;
  height: 28px;
  background: transparent;
  cursor: pointer;
  margin: 0;
}

input[type="range"]::-webkit-slider-runnable-track {
  height: 2px;
  background: linear-gradient(
    90deg,
    var(--accent) 0 var(--p, 44%),
    rgba(25, 23, 16, 0.2) var(--p, 44%) 100%
  );
}

input[type="range"]::-webkit-slider-thumb {
  -webkit-appearance: none;
  margin-top: -10px;
  width: 12px;
  height: 22px;
  border: 1px solid var(--ink);
  border-radius: var(--r);
  background: var(--ink);
}

input[type="range"]:focus-visible::-webkit-slider-thumb {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

input[type="range"]::-moz-range-track {
  height: 2px;
  background: rgba(25, 23, 16, 0.2);
}

input[type="range"]::-moz-range-progress {
  height: 2px;
  background: var(--accent);
}

input[type="range"]::-moz-range-thumb {
  width: 12px;
  height: 22px;
  border: 1px solid var(--ink);
  border-radius: var(--r);
  background: var(--ink);
}

.ticks {
  position: relative;
  height: 16px;
  margin-top: 4px;
  font-family: var(--mono);
  font-size: 10px;
  color: var(--ink-3);
}

.ticks span {
  position: absolute;
  top: 0;
  transform: translateX(-50%);
  white-space: nowrap;
}

.ticks .hot { color: var(--accent); }

.stats {
  margin: 20px 0;
  border-top: 1px solid var(--rule-strong);
}

.stat-row {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 12px;
  padding: 9px 0;
  border-bottom: 1px solid var(--rule);
  font-size: 13px;
  color: var(--ink-3);
}

.stat-row b {
  font-family: var(--mono);
  font-weight: 600;
  font-size: 13px;
  color: var(--ink);
}

.stat-row b.danger { color: var(--bad); }

.actions { display: flex; gap: 10px; }

.btn {
  flex: 1;
  font-family: var(--sans);
  font-size: 13px;
  font-weight: 600;
  letter-spacing: 0.05em;
  color: var(--paper);
  padding: 12px 18px;
  border: 1px solid var(--ink);
  border-radius: var(--r);
  background: var(--ink);
  cursor: pointer;
  transition: background 0.15s, border-color 0.15s, color 0.15s, transform 0.1s;
}

.btn:hover { background: var(--accent); border-color: var(--accent); }
.btn:active { transform: translateY(1px); }

.btn.ghost {
  flex: 0 0 auto;
  color: var(--ink);
  background: transparent;
  border: 1px solid var(--ink);
}

.btn.ghost:hover { background: var(--ink); color: var(--paper); }

.btn:focus-visible,
.lang-toggle:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

.hint {
  margin: 14px 0 0;
  font-size: 12.5px;
  line-height: 1.7;
  color: var(--ink-3);
}

.hint strong { color: var(--accent); font-weight: 600; }

/* ---------- explainer ---------- */

.explain {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(230px, 1fr));
  gap: 16px;
  margin-top: 30px;
}

.explain .card { padding: 20px; }

.explain h3 {
  margin: 0 0 8px;
  font-family: var(--serif);
  font-weight: 600;
  font-size: 17px;
}

.explain p { margin: 0; font-size: 13.5px; color: var(--ink-2); }

.explain code {
  font-family: var(--mono);
  font-size: 0.9em;
  color: var(--accent);
  background: var(--accent-soft);
  border: 1px solid rgba(192, 64, 32, 0.22);
  border-radius: var(--r);
  padding: 1px 6px;
  white-space: nowrap;
}

.callout {
  grid-column: 1 / -1;
  padding: 20px 22px;
  border: 1px solid var(--rule-strong);
  border-left: 3px solid var(--accent);
  border-radius: var(--r);
  background: #f8f2e3;
}

.callout h3 { color: var(--accent); }

/* ---------- knowledge drawer ---------- */

.knowledge { margin-top: 24px; }

.knowledge details {
  border: 1px solid var(--rule-strong);
  border-radius: var(--r);
  background: var(--card);
  overflow: hidden;
}

.knowledge summary {
  cursor: pointer;
  padding: 15px 18px;
  font-family: var(--serif);
  font-weight: 600;
  font-size: 15px;
  color: var(--ink);
  list-style: none;
  user-select: none;
}

.knowledge summary::-webkit-details-marker { display: none; }

.knowledge summary::after {
  content: "+";
  float: right;
  font-family: var(--mono);
  font-weight: 600;
  color: var(--accent);
}

.knowledge details[open] summary::after { content: "−"; }

.knowledge summary:hover { background: rgba(25, 23, 16, 0.03); }

.kn-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(230px, 1fr));
  gap: 12px;
  padding: 4px 18px 18px;
  border-top: 1px solid var(--rule);
  padding-top: 16px;
}

.kn-item {
  border: 1px dashed var(--rule);
  border-radius: var(--r);
  padding: 12px 14px;
  font-size: 13px;
  line-height: 1.7;
  color: var(--ink-2);
  background: rgba(251, 247, 236, 0.6);
}

.kn-item b {
  display: block;
  margin-bottom: 5px;
  font-family: var(--serif);
  font-weight: 600;
  font-size: 14px;
  color: var(--ink);
}

.kn-item code {
  font-family: var(--mono);
  font-size: 0.92em;
  color: var(--accent);
}

/* ---------- gravity calculator ---------- */

.calc {
  margin-top: 24px;
  border: 1px solid var(--rule-strong);
  border-radius: var(--r);
  background: var(--card);
  padding: 20px 22px;
}

.calc h3 {
  margin: 0 0 14px;
  font-family: var(--serif);
  font-weight: 600;
  font-size: 17px;
}

.calc-grid {
  display: grid;
  grid-template-columns: 1fr 1fr 1.2fr;
  gap: 12px;
}

.calc-out {
  margin-top: 14px;
  padding-top: 14px;
  border-top: 1px dashed var(--rule);
}

.calc-f {
  font-family: var(--mono);
  font-weight: 600;
  font-size: 26px;
  color: var(--accent);
  letter-spacing: 0.01em;
}

.calc-cmp {
  margin-top: 4px;
  font-size: 13.5px;
  color: var(--ink-2);
}

.calc-acc {
  margin-top: 6px;
  font-family: var(--mono);
  font-size: 11.5px;
  color: var(--ink-3);
}

@media (max-width: 700px) {
  .calc-grid { grid-template-columns: 1fr; }
}

/* ---------- footer ---------- */

.site-footer {
  max-width: 1120px;
  margin: 0 auto;
  padding: 18px 24px 48px;
  border-top: 1px solid var(--rule-strong);
  font-family: var(--mono);
  font-size: 11px;
  color: var(--ink-3);
}

/* ---------- light figure plate (pendulum / collision) ---------- */

.stage-card.light {
  background: #f7f1e2;
}

.stage-card.light .hud-badge,
.stage-card.light .hud-speed {
  color: var(--ink);
  background: rgba(251, 247, 236, 0.86);
  border-color: var(--rule-strong);
}

.stage-card.light .hud-badge::before { background: var(--ink); }
.stage-card.light .hud-badge.s-circle::before { background: #3f7a4f; }
.stage-card.light .hud-badge.s-warn::before { background: var(--warn); }
.stage-card.light .hud-badge.s-bad::before { background: var(--bad); }
.stage-card.light .hud-badge.s-real::before { background: #3f7a4f; }
.stage-card.light .hud-badge.s-virtual::before { background: var(--warn); }
.stage-card.light .hud-badge.s-none::before { background: var(--bad); }
.stage-card.light .hud-badge.s-sharp::before { background: #3f7a4f; }
.stage-card.light .hud-badge.s-soft::before { background: var(--warn); }
.stage-card.light .hud-badge.s-blob::before { background: var(--bad); }

/* ---------- form fields ---------- */

.field { margin-bottom: 14px; }

.field > label {
  display: block;
  margin-bottom: 6px;
  font-family: var(--mono);
  font-size: 10px;
  font-weight: 600;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--ink-3);
}

select.control-select {
  width: 100%;
  font-family: var(--sans);
  font-size: 13px;
  color: var(--ink);
  background: var(--card);
  border: 1px solid var(--rule-strong);
  border-radius: var(--r);
  padding: 8px 10px;
  cursor: pointer;
}

select.control-select:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

.grid2 {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 10px 14px;
}

.preset-row {
  display: flex;
  gap: 8px;
  margin: 0 0 18px;
}

.preset-row .btn {
  flex: 1;
  font-size: 12.5px;
  padding: 9px 4px;
}

.preset-row .btn.active {
  background: var(--ink);
  color: var(--paper);
  border-color: var(--ink);
}

/* ---------- value bars ---------- */

.bars {
  margin: 14px 0 2px;
  display: flex;
  flex-direction: column;
  gap: 9px;
}

.bar-row {
  display: grid;
  grid-template-columns: 54px 1fr 76px;
  align-items: center;
  gap: 8px;
  font-family: var(--mono);
  font-size: 10.5px;
  color: var(--ink-3);
}

.bar-row b {
  text-align: right;
  font-weight: 600;
  color: var(--ink);
}

.bar-track {
  height: 8px;
  background: rgba(25, 23, 16, 0.08);
  border: 1px solid var(--rule);
  border-radius: 1px;
  overflow: hidden;
}

.bar-fill {
  height: 100%;
  background: var(--ink);
  transition: width 0.2s ease;
}

.bar-fill.accent { background: var(--accent); }

/* ---------- responsive ---------- */

@media (max-width: 900px) {
  .lab { grid-template-columns: 1fr; }
  .panel { position: static; }
  .stage-card { aspect-ratio: 1 / 1; min-height: 0; }
  .hero { padding: 42px 0 32px; }
}

@media (max-width: 520px) {
  main { padding: 6px 16px 48px; }
  .site-header { padding: 18px 16px 12px; }
  .panel { padding: 18px; }
  .topic-card { min-height: 0; }
  .thumb { height: 110px; }
  .explain { grid-template-columns: 1fr; }
}

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  .topic-card, .btn { transition: none; }
}

/* sfx toggle */
.lang-toggle.sfx-toggle.off {
  color: var(--ink-3);
  border-style: dashed;
}