/* ============================================================
   Layout concept: one measured column under a quiet sticky masthead; each
   page opens with a "quote line" (mono figures, like a terminal ticker) and
   then numbered steps. Structure is carried by type and hairlines, cards only
   where an object is genuinely separate.
   ============================================================ */
:root {
  color-scheme: light;
  /* color */
  --paper: #f6f8f9;
  --panel: #ffffff;
  --ink: #18202a;
  --ink-soft: #46525e;
  --muted: #74818c;
  --rule: #dfe5ea;
  --rule-soft: #edf1f4;
  --accent: #1f5f8b;
  --accent-dim: #e8f0f6;
  --on-accent: #ffffff;
  --s-2: #9aa7b2;
  --s-3: #c06b3e;
  --pos: #2f7d5b;
  --neg: #b5473a;
  --shadow: 0 1px 2px rgba(24, 32, 42, .04), 0 8px 24px rgba(24, 32, 42, .05);
  /* type */
  --display: "Source Serif 4", "Songti SC", "STSong", Georgia, serif;
  --sans: "IBM Plex Sans", -apple-system, BlinkMacSystemFont, "Segoe UI", "PingFang SC", "Microsoft YaHei", sans-serif;
  --mono: "IBM Plex Mono", ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  /* layout */
  --maxw: 1120px;
}
/* dark is the default look; light only when the visitor picks it */
:root:not([data-theme="light"]) {
  color-scheme: dark;
  --paper: #12161b; --panel: #192028; --ink: #e2e8ed; --ink-soft: #b4bfc8; --muted: #8995a0;
  --rule: #29323b; --rule-soft: #212930; --accent: #78b6de; --accent-dim: #1c2a36; --on-accent: #12161b;
  --s-2: #6c7a86; --s-3: #db8c61; --pos: #5cb88d; --neg: #e07a6c;
  --shadow: 0 1px 2px rgba(0, 0, 0, .3), 0 8px 24px rgba(0, 0, 0, .25);
}

*, *::before, *::after { box-sizing: border-box; }
[hidden] { display: none !important; }
html { scroll-behavior: smooth; scroll-padding-top: calc(72px + env(safe-area-inset-top, 0px)); }
body {
  margin: 0; background: var(--paper); color: var(--ink);
  font-family: var(--sans); font-size: 15.5px; line-height: 1.62;
  -webkit-font-smoothing: antialiased; font-variant-numeric: tabular-nums;
}
a { color: var(--accent); text-decoration-thickness: 1px; text-underline-offset: 3px; }
:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; border-radius: 3px; }
.wrap {
  max-width: var(--maxw); margin: 0 auto;
  padding-left: max(20px, env(safe-area-inset-left, 0px));
  padding-right: max(20px, env(safe-area-inset-right, 0px));
}

/* ---------- masthead ---------- */
body::before { content: ""; position: fixed; inset: 0 0 auto; height: env(safe-area-inset-top, 0px); z-index: 10; background: var(--paper); }
.site-head {
  position: sticky; top: env(safe-area-inset-top, 0px); z-index: 9;
  background: color-mix(in srgb, var(--paper) 88%, transparent);
  backdrop-filter: saturate(1.4) blur(10px); -webkit-backdrop-filter: saturate(1.4) blur(10px);
  border-bottom: 1px solid var(--rule);
}
.site-head .wrap { display: flex; align-items: center; gap: 24px; min-height: 58px; }
.brand { display: flex; align-items: baseline; gap: 10px; color: var(--ink); text-decoration: none; white-space: nowrap; }
.brand b { font-family: var(--display); font-size: 1.12rem; font-weight: 600; letter-spacing: -0.01em; }
.brand span { font-family: var(--mono); font-size: .72rem; color: var(--muted); }
.nav { display: flex; gap: 4px; margin-left: auto; }
.nav a {
  color: var(--ink-soft); text-decoration: none; font-size: .9rem; padding: 6px 10px; border-radius: 6px;
}
.nav a:hover { color: var(--ink); background: var(--rule-soft); }
.nav a[aria-current="page"] { color: var(--ink); font-weight: 600; box-shadow: inset 0 -2px 0 var(--accent); border-radius: 0; }
.tools { display: flex; align-items: center; gap: 8px; }
.tool {
  font: 600 .74rem var(--mono); color: var(--ink-soft); background: transparent;
  border: 1px solid var(--rule); border-radius: 8px; padding: 0 8px; height: 32px; cursor: pointer; min-width: 32px;
}
.tool:hover { border-color: var(--accent); color: var(--accent); }
.lang { display: inline-flex; align-items: center; height: 32px; padding: 2px; gap: 2px; border: 1px solid var(--rule); border-radius: 8px; background: var(--panel); }
.lang button { font: 500 .76rem var(--sans); letter-spacing: .02em; color: var(--muted); background: transparent; border: 0; border-radius: 6px; padding: 0 10px; height: 26px; cursor: pointer; }
.lang button:hover { color: var(--ink); }
.lang button[aria-pressed="true"] { background: var(--ink); color: var(--paper); cursor: default; }
@media (max-width: 720px) {
  .site-head .wrap { flex-wrap: wrap; gap: 4px 12px; padding-block: 8px; }
  .brand span { display: none; }
  .nav { order: 3; width: 100%; margin-left: -10px; overflow-x: auto; }
  .tools { margin-left: auto; }
}

/* ---------- page opener ---------- */
.opener { padding-block: 56px 8px; }
.eyebrow { margin: 0 0 10px; font: 600 .72rem var(--mono); text-transform: uppercase; letter-spacing: .12em; color: var(--accent); }
.title {
  margin: 0; font-family: var(--display); font-weight: 600; letter-spacing: -0.022em;
  font-size: clamp(2rem, 4.6vw, 3.1rem); line-height: 1.08; text-wrap: balance; max-width: 22ch;
}
.lede { margin: 18px 0 0; font-size: 1.08rem; color: var(--ink-soft); max-width: 62ch; text-wrap: pretty; }

/* quote line: figures in mono, like a terminal ticker */
.quote {
  display: flex; flex-wrap: wrap; gap: 14px 40px; margin-top: 32px; padding-block: 18px;
  border-top: 1px solid var(--ink); border-bottom: 1px solid var(--rule);
}
.q { display: flex; flex-direction: column; gap: 2px; min-width: 0; }
.q-v { font: 500 1.4rem/1.2 var(--mono); letter-spacing: -0.02em; color: var(--ink); }
.q-k { font-size: .75rem; color: var(--muted); letter-spacing: .02em; }
@media (max-width: 640px) { .quote { gap: 12px 26px; } .q-v { font-size: 1.12rem; } }

/* in-page index */
.toc { display: flex; flex-wrap: wrap; gap: 6px 18px; margin-top: 14px; font-size: .84rem; }
.toc a { color: var(--muted); text-decoration: none; }
.toc a:hover { color: var(--accent); }
.toc a b { font: 500 .72rem var(--mono); color: var(--accent); margin-right: 5px; }

/* ---------- blocks ---------- */
.section-label { margin: 0 0 14px; font: 600 .72rem var(--mono); text-transform: uppercase; letter-spacing: .1em; color: var(--muted); }
.findings { margin-top: 40px; }
.cards3 { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 16px; }
.finding { min-width: 0; padding: 20px 22px; background: var(--panel); border: 1px solid var(--rule); border-radius: 10px; }
.finding-label { display: block; font: 600 .7rem var(--mono); letter-spacing: .08em; text-transform: uppercase; color: var(--accent); margin-bottom: 8px; }
.finding-q { margin: 0 0 8px; font-family: var(--display); font-size: 1.12rem; font-weight: 600; line-height: 1.35; }
.finding-a { margin: 0; font-size: .9rem; color: var(--ink-soft); }
.finding-a .n { font-family: var(--mono); font-size: .86rem; color: var(--ink); }

.steps { display: flex; flex-direction: column; gap: 20px; margin-top: 40px; }
.row2 { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 20px; }
.step { min-width: 0; background: var(--panel); border: 1px solid var(--rule); border-radius: 12px; padding: 26px 26px 22px; box-shadow: var(--shadow); }
.step-head { display: grid; grid-template-columns: auto 1fr auto; gap: 14px; align-items: start; margin-bottom: 18px; }
.step-no { font: 500 .78rem var(--mono); color: var(--accent); padding-top: 5px; }
.step-title { margin: 0; font-family: var(--display); font-size: 1.32rem; font-weight: 600; letter-spacing: -0.012em; line-height: 1.3; text-wrap: balance; }
.step-lead { margin: 6px 0 0; font-size: .92rem; color: var(--ink-soft); max-width: 72ch; }
.hint { margin: 14px 0 0; padding: 10px 14px; background: var(--accent-dim); border-radius: 8px; font-size: .86rem; color: var(--ink-soft); }
.sub { margin: 22px 0 10px; font: 600 .7rem var(--mono); letter-spacing: .08em; text-transform: uppercase; color: var(--muted); }
.note { font-size: .8rem; color: var(--muted); margin: 8px 0 0; }

/* segmented control */
.seg { display: inline-flex; border: 1px solid var(--rule); border-radius: 8px; padding: 2px; gap: 2px; flex-wrap: wrap; }
.seg button {
  font: 500 .78rem var(--sans); color: var(--ink-soft); background: transparent; border: 0;
  padding: 5px 11px; border-radius: 6px; cursor: pointer; white-space: nowrap;
}
.seg button:hover { color: var(--ink); }
.seg button[aria-pressed="true"] { background: var(--accent); color: var(--on-accent); }

/* term with explanation (hover or focus) */
.term { position: relative; border-bottom: 1px dotted var(--muted); cursor: help; }
.term .pop {
  position: absolute; left: 50%; bottom: calc(100% + 8px); transform: translateX(-50%); width: max-content; max-width: min(260px, 70vw);
  background: var(--ink); color: var(--paper); font: 400 .78rem/1.5 var(--sans); padding: 8px 10px; border-radius: 6px;
  display: none; pointer-events: none; z-index: 5; text-transform: none; letter-spacing: 0; white-space: normal;
}
.term:hover .pop, .term:focus .pop { display: block; }   /* display, not opacity: hidden tooltips must not widen the page on phones */

/* tables */
.scroll { overflow-x: auto; min-width: 0; }
table { border-collapse: collapse; width: 100%; font-size: .88rem; }
th, td { padding: 9px 10px; text-align: right; border-bottom: 1px solid var(--rule-soft); white-space: nowrap; }
td { font-family: var(--mono); font-size: .84rem; }
th:first-child, td:first-child { text-align: left; font-family: var(--sans); }
th { font-weight: 600; color: var(--muted); font-size: .72rem; letter-spacing: .04em; }
tr.total td { border-top: 1px solid var(--ink); border-bottom: 0; font-weight: 600; }

/* charts */
.chart { position: relative; width: 100%; }
.chart svg { display: block; width: 100%; overflow: visible; }
.chart text { fill: var(--muted); font: 11px var(--mono); }
.chart .lbl { font: 600 11.5px var(--sans); }
.tip {
  position: absolute; pointer-events: none; background: var(--panel); border: 1px solid var(--rule); border-radius: 8px;
  padding: 7px 10px; font: 12px/1.6 var(--mono); white-space: nowrap; display: none; z-index: 2; color: var(--ink); box-shadow: var(--shadow);
}
.tip .sw { display: inline-block; width: 8px; height: 8px; border-radius: 50%; margin-right: 6px; }
.tip .d { color: var(--muted); }

.limits { margin-top: 44px; }
.limits ul { margin: 0; padding-left: 20px; color: var(--ink-soft); font-size: .92rem; max-width: 80ch; }
.limits li { margin-bottom: 6px; }

/* ---------- footer ---------- */
.site-foot { margin-top: 64px; border-top: 1px solid var(--rule); padding-block: 24px 40px; }
.site-foot .wrap { display: flex; flex-wrap: wrap; gap: 10px 28px; align-items: baseline; font-size: .8rem; color: var(--muted); }
.site-foot .links { display: flex; gap: 18px; margin-left: auto; }
.site-foot a { color: var(--ink-soft); text-decoration: none; }
.site-foot a:hover { color: var(--accent); }

.err { color: var(--neg); }

@media (max-width: 980px) { .cards3, .row2 { grid-template-columns: minmax(0, 1fr); } }
@media (max-width: 640px) {
  body { font-size: 15px; }
  .opener { padding-block: 36px 4px; }
  .step { padding: 20px 16px 16px; border-radius: 10px; }
  .step-head { grid-template-columns: auto 1fr; }
  .step-head .seg { grid-column: 1 / -1; }
}
@media (prefers-reduced-motion: reduce) { html { scroll-behavior: auto; } * { transition: none !important; animation: none !important; } }

/* Chinese: system serif CJK fallbacks (SimSun on Windows) look dated at display sizes,
   so headings switch to the sans stack, which maps to PingFang / Microsoft YaHei */
html[lang="zh-CN"] .title, html[lang="zh-CN"] .step-title, html[lang="zh-CN"] .finding-q,
html[lang="zh-CN"] .card h2, html[lang="zh-CN"] .empty h2 { font-family: var(--sans); font-weight: 600; letter-spacing: 0; }
/* CJK titles break only at punctuation, so a phrase never splits mid-word */
html[lang="zh-CN"] .title, html[lang="zh-CN"] .step-title { word-break: keep-all; overflow-wrap: anywhere; }