/* ==========================================================================
   USDBOND — 设计系统
   零外部字体请求（系统字体栈，含 CJK 回退），零框架，纯 CSS 变量驱动。
   深色为品牌默认，浅色跟随系统偏好。所有正文对比度 ≥ WCAG AA。
   ========================================================================== */

:root {
  /* 深色（默认） */
  --bg: #080b11;
  --bg-elev: #0f141d;
  --bg-elev-2: #161d29;
  --fg: #d7dee9;
  --fg-strong: #f4f7fb;
  --fg-muted: #8f9cb0;
  --line: #232c3b;
  --line-soft: #1a2230;

  --accent: #dcae5f;
  --accent-soft: #f0cf95;
  --accent-dim: rgba(220, 174, 95, 0.14);
  --accent-line: rgba(220, 174, 95, 0.32);

  --positive: #4fd1a5;
  --focus: #7cc4ff;

  --shadow: 0 1px 2px rgba(0, 0, 0, 0.4), 0 12px 32px -12px rgba(0, 0, 0, 0.7);
  --radius: 10px;
  --radius-sm: 6px;

  --wrap: 1180px;
  --measure: 72ch;

  --font: -apple-system, BlinkMacSystemFont, "Segoe UI", "Noto Sans SC", "PingFang SC",
    "Hiragino Sans GB", "Microsoft YaHei", "Source Han Sans SC", Roboto, "Helvetica Neue", Arial,
    "Noto Sans", sans-serif;
  --font-mono: ui-monospace, SFMono-Regular, "SF Mono", "Cascadia Code", Menlo, Consolas,
    "Liberation Mono", monospace;
  --font-num: ui-monospace, "SF Mono", "Cascadia Code", Menlo, Consolas, monospace;
}

@media (prefers-color-scheme: light) {
  :root {
    --bg: #fcfbf8;
    --bg-elev: #ffffff;
    --bg-elev-2: #f5f3ee;
    --fg: #2b3038;
    --fg-strong: #12151a;
    --fg-muted: #5d6675;
    --line: #e2ded4;
    --line-soft: #ece9e1;

    --accent: #8a6212;
    --accent-soft: #6d4d0d;
    --accent-dim: rgba(138, 98, 18, 0.09);
    --accent-line: rgba(138, 98, 18, 0.28);

    --positive: #0f7a58;
    --focus: #0b5cad;

    --shadow: 0 1px 2px rgba(20, 20, 15, 0.06), 0 12px 28px -14px rgba(20, 20, 15, 0.18);
  }
}

/* --------------------------------------------------------------- 基础 */

*,
*::before,
*::after {
  box-sizing: border-box;
}

html {
  -webkit-text-size-adjust: 100%;
  scroll-behavior: smooth;
  scroll-padding-top: 5.5rem;
}

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

body {
  margin: 0;
  background: var(--bg);
  color: var(--fg);
  font-family: var(--font);
  font-size: clamp(1rem, 0.96rem + 0.2vw, 1.0625rem);
  line-height: 1.75;
  text-rendering: optimizeLegibility;
  -webkit-font-smoothing: antialiased;
  font-kerning: normal;
  overflow-wrap: break-word;
}

/* 顶部一层极淡的品牌光晕，纯装饰 */
body::before {
  content: "";
  position: fixed;
  inset: 0 0 auto 0;
  height: 420px;
  pointer-events: none;
  z-index: 0;
  background:
    radial-gradient(60rem 22rem at 50% -6rem, var(--accent-dim), transparent 70%),
    linear-gradient(var(--line-soft), transparent);
  opacity: 0.9;
}

body > * {
  position: relative;
  z-index: 1;
}

img,
svg {
  max-width: 100%;
  height: auto;
}

.wrap {
  width: 100%;
  max-width: var(--wrap);
  margin-inline: auto;
  padding-inline: clamp(1.125rem, 4vw, 2.5rem);
}

.sep {
  color: var(--fg-muted);
  margin-inline: 0.5em;
  font-weight: 300;
}

/* 键盘焦点始终可见 */
:focus-visible {
  outline: 2px solid var(--focus);
  outline-offset: 3px;
  border-radius: 3px;
}

/* 跳到主要内容 */
.skip {
  position: absolute;
  left: -9999px;
  top: 0;
  z-index: 100;
  padding: 0.75rem 1.25rem;
  background: var(--accent);
  color: #0a0d13;
  font-weight: 650;
  text-decoration: none;
}

.skip:focus {
  left: 0;
}

/* --------------------------------------------------------------- 页头 */

.site-header {
  position: sticky;
  top: 0;
  z-index: 50;
  background: color-mix(in srgb, var(--bg) 86%, transparent);
  backdrop-filter: saturate(150%) blur(14px);
  border-bottom: 1px solid var(--line);
}

@supports not (backdrop-filter: blur(1px)) {
  .site-header {
    background: var(--bg);
  }
}

.site-header__inner {
  display: flex;
  align-items: center;
  gap: clamp(1rem, 2.5vw, 2rem);
  min-height: 4.25rem;
  flex-wrap: wrap;
}

.brand {
  display: inline-flex;
  align-items: center;
  gap: 0.65rem;
  text-decoration: none;
  color: var(--fg-strong);
  flex: 0 0 auto;
}

.brand__mark {
  display: grid;
  place-items: center;
  width: 2.1rem;
  height: 2.1rem;
  border-radius: 8px;
  background: linear-gradient(150deg, var(--accent-soft), var(--accent));
  color: #1a1206;
  font-weight: 800;
  font-size: 1.05rem;
  letter-spacing: -0.02em;
  flex: 0 0 auto;
}

.brand__mark--lg {
  width: 3.25rem;
  height: 3.25rem;
  border-radius: 12px;
  font-size: 1.6rem;
}

.brand__text {
  display: flex;
  flex-direction: column;
  line-height: 1.15;
}

.brand__text strong {
  font-size: 1.0625rem;
  letter-spacing: -0.01em;
}

.brand__tag {
  font-size: 0.6875rem;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--fg-muted);
  font-weight: 500;
}

.site-nav {
  flex: 1 1 auto;
  min-width: 0;
}

.site-nav ul {
  display: flex;
  gap: 0.25rem;
  margin: 0;
  padding: 0;
  list-style: none;
  overflow-x: auto;
  scrollbar-width: none;
}

.site-nav ul::-webkit-scrollbar {
  display: none;
}

.site-nav a {
  display: block;
  padding: 0.4rem 0.7rem;
  border-radius: var(--radius-sm);
  color: var(--fg-muted);
  text-decoration: none;
  font-size: 0.9rem;
  font-weight: 500;
  white-space: nowrap;
  transition: color 0.15s, background-color 0.15s;
}

.site-nav a:hover {
  color: var(--fg-strong);
  background: var(--bg-elev-2);
}

.site-nav a[aria-current="page"] {
  color: var(--accent);
  background: var(--accent-dim);
}

.lang-switch {
  flex: 0 0 auto;
  padding: 0.4rem 0.8rem;
  border: 1px solid var(--line);
  border-radius: 999px;
  color: var(--fg-muted);
  text-decoration: none;
  font-size: 0.82rem;
  font-weight: 550;
  white-space: nowrap;
  transition: border-color 0.15s, color 0.15s;
}

.lang-switch:hover {
  color: var(--accent);
  border-color: var(--accent-line);
}

/* --------------------------------------------------------------- 面包屑 */

.crumbs {
  max-width: var(--wrap);
  margin-inline: auto;
  padding: 1.5rem clamp(1.125rem, 4vw, 2.5rem) 0;
}

.crumbs ol {
  display: flex;
  gap: 0.5rem;
  margin: 0;
  padding: 0;
  list-style: none;
  font-size: 0.8rem;
  color: var(--fg-muted);
}

.crumbs li + li::before {
  content: "/";
  margin-right: 0.5rem;
  color: var(--line);
}

.crumbs a {
  color: var(--fg-muted);
  text-decoration: none;
}

.crumbs a:hover {
  color: var(--accent);
}

/* ------------------------------------------------------------- 孵化关系条 */

.incubator-bar {
  border-bottom: 1px solid var(--line-soft);
  background: var(--bg-elev);
}

.incubator-bar__inner {
  display: flex;
  align-items: center;
  min-height: 2.1rem;
  padding-block: 0.3rem;
}

.incubator-bar__text {
  margin: 0;
  font-size: 0.76rem;
  line-height: 1.5;
  color: var(--fg-muted);
  letter-spacing: 0.01em;
}

.incubator-bar__text a {
  color: var(--accent);
  text-decoration: none;
  border-bottom: 1px solid var(--accent-line);
  transition: border-color 0.15s;
}

.incubator-bar__text a:hover {
  border-bottom-color: var(--accent);
}

.incubator-note {
  max-width: 78ch;
  margin: 0 0 1.5rem;
  font-size: 0.76rem;
  line-height: 1.65;
  color: var(--fg-muted);
  opacity: 0.9;
}

.gateway__incubator {
  margin: 2.5rem 0 0;
  padding-top: 1.5rem;
  border-top: 1px solid var(--line);
  font-size: 0.78rem;
  color: var(--fg-muted);
}

.gateway__incubator a {
  color: var(--accent);
  text-decoration: none;
}

/* --------------------------------------------------------------- 页面头 */

.page {
  padding-block: clamp(1.75rem, 4vw, 3rem) clamp(3rem, 7vw, 5.5rem);
}

.page__head {
  max-width: var(--measure);
  padding-block: clamp(1rem, 3vw, 2rem);
}

.kicker {
  margin: 0 0 0.85rem;
  font-size: 0.7rem;
  font-weight: 700;
  letter-spacing: 0.16em;
  text-transform: uppercase;
  color: var(--accent);
}

.page h1,
.gateway h1 {
  margin: 0 0 1rem;
  font-size: clamp(2rem, 1.35rem + 3.1vw, 3.4rem);
  line-height: 1.1;
  letter-spacing: -0.025em;
  color: var(--fg-strong);
  font-weight: 700;
  text-wrap: balance;
}

.token {
  font-family: var(--font-num);
  font-size: 0.55em;
  color: var(--accent);
  letter-spacing: 0;
  vertical-align: 0.28em;
  padding-left: 0.15em;
}

.lead {
  margin: 0;
  font-size: clamp(1.0625rem, 1rem + 0.35vw, 1.2rem);
  line-height: 1.72;
  color: var(--fg);
  text-wrap: pretty;
}

.meta {
  margin: 1.25rem 0 0;
  font-size: 0.78rem;
  color: var(--fg-muted);
  font-family: var(--font-num);
}

/* --------------------------------------------------------------- 数据卡 */

.stats {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(11rem, 1fr));
  gap: 1px;
  margin: 1rem 0 0;
  padding: 0;
  background: var(--line);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  overflow: hidden;
}

.stat {
  margin: 0;
  padding: 1.4rem 1.4rem 1.5rem;
  background: var(--bg-elev);
}

.stat__label {
  font-size: 0.72rem;
  font-weight: 650;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--fg-muted);
}

.stat__value {
  margin: 0.4rem 0 0.3rem;
  font-family: var(--font-num);
  font-size: clamp(1.5rem, 1.2rem + 1.1vw, 2rem);
  line-height: 1.1;
  font-weight: 600;
  color: var(--accent);
  letter-spacing: -0.02em;
  font-variant-numeric: tabular-nums;
}

.stat__note {
  margin: 0;
  font-size: 0.8rem;
  line-height: 1.5;
  color: var(--fg-muted);
}

/* --------------------------------------------------------------- 正文布局 */

.page__body {
  margin-top: clamp(2rem, 5vw, 3.25rem);
  display: grid;
  grid-template-columns: minmax(0, 1fr);
  gap: clamp(2rem, 4vw, 3.5rem);
}

@media (min-width: 62rem) {
  .page__body:has(.toc) {
    grid-template-columns: 15rem minmax(0, 1fr);
  }
  .toc {
    grid-column: 1;
  }
  .prose {
    grid-column: 2;
  }
}

/* 目录 */
.toc {
  align-self: start;
}

@media (min-width: 62rem) {
  .toc {
    position: sticky;
    top: 6rem;
    max-height: calc(100vh - 8rem);
    overflow-y: auto;
  }
}

.toc__title {
  margin: 0 0 0.85rem;
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--fg-muted);
}

.toc__list {
  margin: 0;
  padding: 0 0 0 0.9rem;
  list-style: none;
  border-left: 1px solid var(--line);
}

.toc__item {
  margin: 0;
}

.toc__item a {
  display: block;
  padding: 0.28rem 0;
  font-size: 0.86rem;
  line-height: 1.45;
  color: var(--fg-muted);
  text-decoration: none;
  border-left: 2px solid transparent;
  margin-left: calc(-0.9rem - 1px);
  padding-left: calc(0.9rem - 1px);
  transition: color 0.15s, border-color 0.15s;
}

.toc__item a:hover {
  color: var(--accent);
  border-left-color: var(--accent-line);
}

.toc__item--h3 a {
  padding-left: calc(0.9rem - 1px + 0.85rem);
  font-size: 0.8rem;
  color: var(--fg-muted);
  opacity: 0.88;
}

/* --------------------------------------------------------------- prose */

.prose {
  max-width: var(--measure);
  min-width: 0;
}

.prose > * + * {
  margin-top: 1.15rem;
}

.prose h2 {
  margin-top: clamp(2.75rem, 5vw, 4rem);
  margin-bottom: 0;
  padding-top: 1.4rem;
  border-top: 1px solid var(--line);
  font-size: clamp(1.4rem, 1.15rem + 1vw, 1.85rem);
  line-height: 1.25;
  letter-spacing: -0.02em;
  color: var(--fg-strong);
  font-weight: 650;
  text-wrap: balance;
}

.prose h3 {
  margin-top: 2.25rem;
  margin-bottom: 0;
  font-size: clamp(1.08rem, 1rem + 0.4vw, 1.28rem);
  line-height: 1.35;
  letter-spacing: -0.01em;
  color: var(--fg-strong);
  font-weight: 650;
  text-wrap: balance;
}

.prose h2 + *,
.prose h3 + * {
  margin-top: 0.85rem;
}

.prose p {
  margin: 0;
  text-wrap: pretty;
}

.prose strong {
  color: var(--fg-strong);
  font-weight: 650;
}

.prose em {
  color: var(--fg);
}

.prose a {
  color: var(--accent);
  text-decoration: underline;
  text-decoration-thickness: 1px;
  text-underline-offset: 0.18em;
  text-decoration-color: var(--accent-line);
  transition: text-decoration-color 0.15s;
}

.prose a:hover {
  text-decoration-color: var(--accent);
}

.prose ul,
.prose ol {
  margin: 0;
  padding-left: 1.35rem;
}

.prose li {
  margin-top: 0.45rem;
}

.prose li::marker {
  color: var(--accent);
}

.prose hr {
  margin-top: 2.5rem;
  border: 0;
  border-top: 1px solid var(--line);
}

/* 锚点 */
.anchor {
  margin-left: 0.4rem;
  font-size: 0.7em;
  color: var(--line);
  text-decoration: none;
  opacity: 0;
  transition: opacity 0.15s, color 0.15s;
}

h2:hover .anchor,
h3:hover .anchor,
.anchor:focus-visible {
  opacity: 1;
  color: var(--accent);
}

/* 引用 = 提示框。正文用 > 标记语义上的"补充说明 / 定义"。 */
.prose blockquote {
  margin-top: 1.65rem;
  margin-bottom: 1.65rem;
  padding: 1.15rem 1.35rem;
  background: var(--bg-elev);
  border: 1px solid var(--line);
  border-left: 3px solid var(--accent);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  color: var(--fg);
}

.prose blockquote > * + * {
  margin-top: 0.75rem;
}

.prose blockquote p {
  margin: 0;
}

.prose blockquote strong {
  color: var(--accent-soft);
}

/* 表格 */
.table-wrap {
  margin-top: 1.65rem;
  overflow-x: auto;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--bg-elev);
  -webkit-overflow-scrolling: touch;
}

.prose table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.885rem;
  line-height: 1.6;
  min-width: 34rem;
}

.prose thead th {
  position: sticky;
  top: 0;
  background: var(--bg-elev-2);
  text-align: left;
  font-weight: 650;
  font-size: 0.78rem;
  letter-spacing: 0.03em;
  text-transform: uppercase;
  color: var(--fg-muted);
  white-space: nowrap;
}

.prose th,
.prose td {
  padding: 0.7rem 0.95rem;
  border-bottom: 1px solid var(--line-soft);
  vertical-align: top;
}

.prose tbody tr:last-child td {
  border-bottom: 0;
}

.prose tbody tr:hover td {
  background: var(--bg-elev-2);
}

.prose td strong {
  color: var(--fg-strong);
}

/* 数字列使用等宽数字对齐 */
.prose td,
.prose th {
  font-variant-numeric: tabular-nums;
}

/* 代码 */
.prose code {
  font-family: var(--font-mono);
  font-size: 0.86em;
  padding: 0.15em 0.38em;
  background: var(--bg-elev-2);
  border: 1px solid var(--line-soft);
  border-radius: 4px;
  color: var(--accent-soft);
}

.prose pre {
  margin-top: 1.65rem;
  padding: 1.15rem 1.25rem;
  overflow-x: auto;
  background: var(--bg-elev);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  line-height: 1.6;
}

.prose pre code {
  padding: 0;
  background: none;
  border: 0;
  font-size: 0.82rem;
  color: var(--fg);
}

/* --------------------------------------------------------------- 问答块 */

/* 问答块：就地嵌在所属章节里，靠分隔线成组，不做折叠 ——
   折叠会让内容在 HTML 里依然存在却显得次要，对 SEO/GEO 没有好处。 */
.qa {
  padding-block: 1.4rem;
  border-bottom: 1px solid var(--line-soft);
}

.qa:last-child {
  border-bottom: 0;
  padding-bottom: 0;
}

.qa__q {
  margin: 0 !important;
  font-size: clamp(1.05rem, 1rem + 0.3vw, 1.18rem) !important;
  color: var(--fg-strong) !important;
  border: 0 !important;
  padding: 0 !important;
}

.qa__q::before {
  content: "Q ";
  font-family: var(--font-num);
  font-size: 0.8em;
  color: var(--accent);
  font-weight: 700;
  letter-spacing: 0.05em;
  margin-right: 0.3em;
}

.qa__a > * + * {
  margin-top: 0.75rem;
}

.qa__a {
  margin-top: 0.7rem;
  color: var(--fg);
}

.qa__a p {
  margin: 0;
}

/* --------------------------------------------------------------- 按钮 */

.btn {
  display: inline-block;
  padding: 0.7rem 1.4rem;
  border: 1px solid var(--line);
  border-radius: 999px;
  background: var(--bg-elev);
  color: var(--fg-strong);
  text-decoration: none;
  font-weight: 600;
  font-size: 0.95rem;
  transition: border-color 0.15s, transform 0.15s, background-color 0.15s;
}

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

.btn--primary {
  background: linear-gradient(150deg, var(--accent-soft), var(--accent));
  border-color: transparent;
  color: #1a1206;
}

/* --------------------------------------------------------------- 页脚 */

.site-footer {
  margin-top: clamp(3rem, 7vw, 5rem);
  border-top: 1px solid var(--line);
  background: var(--bg-elev);
  padding-block: clamp(2.25rem, 5vw, 3.25rem);
}

.footer-nav ul {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem 1.5rem;
  margin: 0 0 1.75rem;
  padding: 0;
  list-style: none;
}

.footer-nav a {
  color: var(--fg-muted);
  text-decoration: none;
  font-size: 0.88rem;
}

.footer-nav a:hover {
  color: var(--accent);
}

.disclaimer {
  max-width: 78ch;
  margin: 0 0 1.5rem;
  padding: 1.1rem 1.25rem;
  font-size: 0.79rem;
  line-height: 1.68;
  color: var(--fg-muted);
  background: var(--bg);
  border: 1px solid var(--line-soft);
  border-radius: var(--radius-sm);
}

.copyright {
  margin: 0;
  font-size: 0.8rem;
  color: var(--fg-muted);
}

.copyright a {
  color: var(--fg-muted);
  text-decoration: underline;
  text-underline-offset: 0.18em;
}

.copyright a:hover {
  color: var(--accent);
}

/* --------------------------------------------------------------- 语言网关 */

.gateway__main {
  display: grid;
  place-items: center;
  min-height: 100vh;
  padding-block: 4rem;
}

.gateway__inner {
  max-width: 46rem;
  text-align: center;
}

.gateway__inner .brand__mark--lg {
  margin: 0 auto 1.5rem;
}

.gateway__tag {
  margin: 0 0 1.75rem;
  font-size: 0.85rem;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--accent);
  font-weight: 600;
}

.gateway__lead {
  max-width: 62ch;
  margin: 0 auto 0.85rem;
  font-size: 0.95rem;
  line-height: 1.7;
  color: var(--fg-muted);
}

.gateway__langs {
  display: flex;
  gap: 0.85rem;
  justify-content: center;
  flex-wrap: wrap;
  margin: 2rem 0 1rem;
}

.gateway__hint {
  margin: 0 0 2.5rem;
  font-size: 0.78rem;
  color: var(--fg-muted);
  min-height: 1.2em;
}

.gateway__hint[hidden] {
  display: none;
}

.gateway__index {
  border-top: 1px solid var(--line);
  padding-top: 1.75rem;
}

.gateway__index ul {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 0.5rem 1.5rem;
  margin: 0;
  padding: 0;
  list-style: none;
}

.gateway__index a {
  color: var(--fg-muted);
  text-decoration: none;
  font-size: 0.85rem;
}

.gateway__index a:hover {
  color: var(--accent);
}

.gateway__index .sep {
  margin-inline: 0.35em;
  color: var(--line);
}

/* --------------------------------------------------------------- 打印 */

@media print {
  .site-header,
  .toc,
  .lang-switch,
  .site-footer,
  .anchor,
  .skip {
    display: none !important;
  }

  body {
    background: #fff;
    color: #000;
    font-size: 11pt;
  }

  body::before {
    display: none;
  }

  .prose a::after {
    content: " (" attr(href) ")";
    font-size: 0.85em;
    color: #444;
  }
}