:root {
  --bg: #f6f7f9;
  --surface: #ffffff;
  --surface-2: #eef0f4;
  --border: #d9dde5;
  --text: #1c2230;
  --muted: #667085;
  --accent: #2563eb;
  --accent-soft: #dbe7ff;
  --good: #15803d;
  --good-soft: #dcfce7;
  --bad: #b91c1c;
  --bad-soft: #fee2e2;
  --warn-soft: #fef3c7;
  --hl: #fff3b0;
  --code-bg: #f1f3f8;
  --tk-kw: #8250df;
  --tk-type: #0b6e99;
  --tk-str: #1a7f37;
  --tk-num: #b35900;
  --tk-com: #7a8294;
  --tk-pre: #b0306b;
  --tk-fn: #1f56c4;
  --mono: "JetBrains Mono", ui-monospace, "SF Mono", Menlo, Consolas, monospace;
  --sans: system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) { --dark: 1; }
}
:root:not([data-theme="light"]) { }
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #0f131a; --surface: #171c26; --surface-2: #1f2633; --border: #2c3545;
    --text: #e6e9f0; --muted: #98a2b3; --accent: #6ea0ff; --accent-soft: #1c2c52;
    --good: #4ade80; --good-soft: #12301e; --bad: #f87171; --bad-soft: #3a1717; --warn-soft: #3a3010;
    --hl: #4a3f10; --code-bg: #12171f;
    --tk-kw: #c4a1ff; --tk-type: #5cc8f0; --tk-str: #7ee0a0; --tk-num: #f0a860; --tk-com: #7f8aa0; --tk-pre: #f08cb8; --tk-fn: #8db8ff;
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #0f131a; --surface: #171c26; --surface-2: #1f2633; --border: #2c3545;
  --text: #e6e9f0; --muted: #98a2b3; --accent: #6ea0ff; --accent-soft: #1c2c52;
  --good: #4ade80; --good-soft: #12301e; --bad: #f87171; --bad-soft: #3a1717; --warn-soft: #3a3010;
  --hl: #4a3f10; --code-bg: #12171f;
  --tk-kw: #c4a1ff; --tk-type: #5cc8f0; --tk-str: #7ee0a0; --tk-num: #f0a860; --tk-com: #7f8aa0; --tk-pre: #f08cb8; --tk-fn: #8db8ff;
  color-scheme: dark;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body { margin: 0; background: var(--bg); color: var(--text); font: 16px/1.6 var(--sans); }
code, pre, kbd { font-family: var(--mono); }
p code, li code, td code, .note code { background: var(--surface-2); padding: 0.1em 0.35em; border-radius: 4px; font-size: 0.9em; }
.muted { color: var(--muted); font-weight: 400; }

/* ---------- layout ---------- */
.app { display: grid; grid-template-columns: 272px minmax(0, 1fr); min-height: 100vh; }
.sidebar {
  position: sticky; top: 0; height: 100vh; overflow-y: auto; padding: 20px 16px;
  background: var(--surface); border-right: 1px solid var(--border); display: flex; flex-direction: column; gap: 16px;
}
.brand { display: flex; gap: 12px; align-items: center; }
.logo { width: 40px; height: 40px; border-radius: 10px; background: var(--accent); color: #fff; display: grid; place-items: center; font: 700 22px var(--mono); }
.brand-title { font-weight: 700; }
.brand-sub { font-size: 13px; color: var(--muted); }
.progress-bar { height: 6px; border-radius: 3px; background: var(--surface-2); overflow: hidden; }
.progress-bar span { display: block; height: 100%; width: 0; background: var(--good); transition: width .3s; }
.progress-text { font-size: 12px; color: var(--muted); margin-top: 6px; }
#nav { display: flex; flex-direction: column; gap: 2px; flex: 1; }
#nav a {
  display: flex; gap: 10px; align-items: center; padding: 8px 10px; border-radius: 8px; color: var(--text); text-decoration: none; font-size: 15px;
}
#nav a:hover { background: var(--surface-2); }
#nav a.active { background: var(--accent-soft); font-weight: 600; }
#nav .num { width: 24px; height: 24px; border-radius: 50%; border: 1.5px solid var(--border); display: grid; place-items: center; font-size: 12px; flex: none; }
#nav a.done .num { background: var(--good); border-color: var(--good); color: #fff; }
.sidebar-foot { display: flex; gap: 8px; flex-wrap: wrap; }
main { min-width: 0; }
.topbar { display: none; }
.scrim { display: none; }
.lesson { max-width: 1040px; margin: 0 auto; padding: 36px 32px 80px; }
.lesson h1 { font-size: 2rem; line-height: 1.2; margin: 0 0 6px; }
.lesson .lead { color: var(--muted); font-size: 1.1rem; margin: 0 0 28px; }
.lesson h2 { margin: 36px 0 8px; font-size: 1.3rem; }
.lesson p, .lesson li { max-width: 72ch; }
.lesson-nav { display: flex; justify-content: space-between; gap: 12px; margin-top: 48px; flex-wrap: wrap; }

@media (max-width: 860px) {
  .app { grid-template-columns: 1fr; }
  .sidebar { position: fixed; z-index: 30; left: 0; top: 0; width: 280px; max-width: 86vw; transform: translateX(-100%); transition: transform .2s; }
  body.menu-open .sidebar { transform: none; }
  body.menu-open .scrim { display: block; position: fixed; inset: 0; background: #0008; z-index: 20; }
  .topbar { display: flex; align-items: center; gap: 12px; padding: 10px 16px; position: sticky; top: 0; z-index: 10; background: var(--surface); border-bottom: 1px solid var(--border); }
  .topbar-title { font-weight: 700; }
  .lesson { padding: 24px 16px 64px; }
}

/* ---------- buttons ---------- */
.btn {
  font: inherit; font-size: 15px; padding: 8px 14px; border-radius: 8px; border: 1px solid var(--border);
  background: var(--surface); color: var(--text); cursor: pointer;
}
.btn:hover { background: var(--surface-2); }
.btn:focus-visible, .stepper:focus-visible, #nav a:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }
.btn.small { padding: 5px 10px; font-size: 13px; }
.btn.primary { background: var(--accent); border-color: var(--accent); color: #fff; }
.btn.primary:hover { filter: brightness(1.08); }
.btn.ghost { background: transparent; }
.btn.good { background: var(--good); border-color: var(--good); color: #fff; }

/* ---------- code ---------- */
.tk-kw { color: var(--tk-kw); font-weight: 600; } .tk-type { color: var(--tk-type); } .tk-str { color: var(--tk-str); }
.tk-num { color: var(--tk-num); } .tk-com { color: var(--tk-com); font-style: italic; } .tk-pre { color: var(--tk-pre); } .tk-fn { color: var(--tk-fn); }
.codeblock { margin: 16px 0; border: 1px solid var(--border); border-radius: 10px; overflow: hidden; background: var(--code-bg); }
.codeblock figcaption { padding: 6px 14px; font-size: 12px; color: var(--muted); border-bottom: 1px solid var(--border); background: var(--surface-2); }
pre.code { margin: 0; padding: 12px 0; overflow-x: auto; font-size: 14px; line-height: 1.7; font-variant-ligatures: none; }
.ln { display: flex; }
.ln-no { width: 38px; flex: none; text-align: right; padding-right: 12px; color: var(--muted); user-select: none; opacity: .7; }
.ln-code { white-space: pre; padding-right: 14px; }
pre.code .ln { display: flex; }

/* ---------- callouts, tables ---------- */
.note { border-left: 4px solid var(--accent); background: var(--accent-soft); padding: 10px 16px; border-radius: 0 8px 8px 0; margin: 16px 0; max-width: 72ch; }
.note.warn { border-color: #d97706; background: var(--warn-soft); }
.lesson table { border-collapse: collapse; margin: 14px 0; font-size: 15px; }
.lesson th, .lesson td { border: 1px solid var(--border); padding: 6px 12px; text-align: left; }
.lesson th { background: var(--surface-2); }
.pipeline { display: flex; flex-wrap: wrap; gap: 8px; align-items: center; margin: 16px 0; }
.pipeline .box { padding: 8px 12px; border: 1px solid var(--border); border-radius: 8px; background: var(--surface); font-size: 14px; }
.pipeline .box small { display: block; color: var(--muted); font: 12px var(--mono); }
.pipeline .arr { color: var(--muted); }

/* ---------- stepper ---------- */
.stepper { margin: 22px 0; border: 1px solid var(--border); border-radius: 12px; background: var(--surface); overflow: hidden; }
.st-head { display: flex; justify-content: space-between; gap: 12px; flex-wrap: wrap; align-items: center; padding: 10px 14px; border-bottom: 1px solid var(--border); background: var(--surface-2); }
.st-title { font-weight: 600; font-size: 14px; }
.st-param { font-size: 13px; display: flex; gap: 8px; align-items: center; }
.st-param input { width: 110px; }
.st-body { display: grid; grid-template-columns: minmax(0, 1fr) minmax(0, 1fr); }
.st-code { font-family: var(--mono); background: var(--code-bg); max-height: 440px; overflow: auto; padding: 8px 0; border-right: 1px solid var(--border); font-size: 14px; line-height: 1.7; font-variant-ligatures: none; position: relative; }
.st-lines .ln { border-left: 3px solid transparent; }
.st-lines .ln.cur { background: var(--hl); border-left-color: #f59e0b; }
.st-lines .ln.prev { background: var(--surface-2); }
.st-side { padding: 12px 14px; display: flex; flex-direction: column; gap: 12px; min-width: 0; }
.st-note { min-height: 3.2em; padding: 8px 12px; border-radius: 8px; background: var(--accent-soft); font-size: 14.5px; line-height: 1.5; }
.st-mem { position: relative; display: flex; flex-direction: column; gap: 8px; }
.st-arrows { position: absolute; left: 0; top: 0; pointer-events: none; overflow: visible; z-index: 2; }
.arrow { fill: none; stroke: var(--accent); stroke-width: 2; }
.arrow.dangling { stroke: var(--bad); stroke-dasharray: 5 4; }
.ah { fill: var(--accent); } .ah.dangling { fill: var(--bad); }
.st-section-label { font-size: 12px; text-transform: uppercase; letter-spacing: .05em; color: var(--muted); font-weight: 600; margin-bottom: 4px; }
.frame { border: 1.5px solid var(--border); border-radius: 8px; padding: 6px 10px 8px; margin-bottom: 8px; background: var(--surface); }
.frame.active { border-color: var(--accent); }
.frame-head { font: 600 13px var(--mono); margin-bottom: 4px; color: var(--muted); }
.frame.active .frame-head { color: var(--accent); }
.var, .block { padding: 3px 0; }
.var-row { display: flex; gap: 8px; align-items: baseline; flex-wrap: wrap; font: 14px var(--mono); }
.v-type { color: var(--tk-type); }
.v-name { font-weight: 700; }
.v-val { background: var(--surface-2); border: 1px solid var(--border); border-radius: 5px; padding: 0 8px; min-width: 2.2em; text-align: center; }
.v-val.ptr { color: var(--accent); }
.v-addr { margin-left: auto; font-size: 11px; color: var(--muted); }
.garbage { color: var(--bad); font-weight: 700; }
.var.changed .v-val, .cell.changed { animation: flash 1.1s ease-out; }
@keyframes flash { 0% { background: var(--hl); box-shadow: 0 0 0 3px #f59e0b88; } 100% { box-shadow: none; } }
.block { border: 1.5px dashed var(--border); border-radius: 8px; padding: 6px 10px 8px; margin-bottom: 8px; background: var(--surface); }
.block.freed { opacity: .65; border-color: var(--bad); }
.block.changed { animation: flash 1.1s ease-out; }
.badge.bad { background: var(--bad-soft); color: var(--bad); font: 600 11px var(--sans); padding: 1px 8px; border-radius: 10px; }
.cells { display: flex; flex-wrap: wrap; gap: 4px; margin-top: 4px; }
.cell { min-width: 42px; text-align: center; border: 1.5px solid var(--border); border-radius: 5px; background: var(--surface-2); font: 14px var(--mono); }
.cell.hl { border-color: var(--accent); }
.c-lbl { display: block; font-size: 10px; color: var(--muted); border-bottom: 1px solid var(--border); padding: 0 4px; }
.c-val { display: block; padding: 2px 6px; min-height: 1.6em; }
.empty { color: var(--muted); font-size: 13px; font-style: italic; }
.st-out pre { margin: 0; padding: 8px 12px; background: #0b0f16; color: #d7e0ee; border-radius: 8px; min-height: 2.6em; font-size: 13.5px; white-space: pre-wrap; }
.st-out .muted { color: #7f8aa0; }
.st-controls { display: flex; gap: 8px; align-items: center; flex-wrap: wrap; padding: 10px 14px; border-top: 1px solid var(--border); background: var(--surface-2); }
.st-scrub { flex: 1; min-width: 100px; }
.st-count { font-size: 13px; color: var(--muted); white-space: nowrap; }
@media (max-width: 700px) {
  .st-body { grid-template-columns: 1fr; }
  .st-code { border-right: 0; border-bottom: 1px solid var(--border); max-height: 280px; }
}
@media (prefers-reduced-motion: reduce) {
  .var.changed .v-val, .cell.changed, .block.changed { animation: none; }
  html { scroll-behavior: auto; }
}

/* ---------- widgets ---------- */
.widget { margin: 22px 0; border: 1px solid var(--border); border-radius: 12px; background: var(--surface); padding: 14px 16px; }
.widget h3 { margin: 0 0 4px; font-size: 1rem; }
.widget .sub { color: var(--muted); font-size: 14px; margin: 0 0 12px; }
.quiz .q { font-weight: 600; margin: 0 0 10px; }
.quiz .opts { display: grid; gap: 8px; }
.quiz .opt { text-align: left; font: 15px var(--mono); padding: 9px 12px; border-radius: 8px; border: 1.5px solid var(--border); background: var(--surface); color: var(--text); cursor: pointer; }
.quiz .opt:hover:not(:disabled) { border-color: var(--accent); }
.quiz .opt.right { background: var(--good-soft); border-color: var(--good); }
.quiz .opt.wrong { background: var(--bad-soft); border-color: var(--bad); }
.quiz .why { margin-top: 10px; font-size: 14.5px; }
.quiz .tag { float: right; font-size: 12px; color: var(--good); font-weight: 600; }
.bits { display: flex; gap: 6px; flex-wrap: wrap; margin-bottom: 12px; }
.bit { width: 44px; height: 54px; border-radius: 8px; border: 1.5px solid var(--border); background: var(--surface-2); font: 600 20px var(--mono); color: var(--text); cursor: pointer; display: flex; flex-direction: column; align-items: center; justify-content: center; }
.bit small { font: 10px var(--sans); color: var(--muted); }
.bit.on { background: var(--accent); border-color: var(--accent); color: #fff; }
.bit.on small { color: #fffc; }
.readouts { display: grid; grid-template-columns: repeat(auto-fit, minmax(130px, 1fr)); gap: 8px; }
.readout { background: var(--surface-2); border-radius: 8px; padding: 8px 12px; }
.readout span { display: block; font-size: 12px; color: var(--muted); }
.readout b { font: 700 18px var(--mono); }
.text-in { font: 16px var(--mono); padding: 8px 10px; border-radius: 8px; border: 1.5px solid var(--border); background: var(--bg); color: var(--text); width: min(100%, 360px); }