/* ================================================================
   THE PHYSICS CODEX — style.css
   thecodex.expert/physics/
   Single external stylesheet. All styles live here. Never inline
   after Session 01. All CSS variables per PHYSICS-CODEX-MASTER-SPEC-v2 Part 4.
   ================================================================ */

:root {
  --bg:          #F7F8FC;
  --bg2:         #EDEEF5;
  --bg3:         #E2E4EF;
  --bg-code:     #0F1117;
  --white:       #ffffff;
  --ink:         #0D0F1A;
  --ink2:        #2C3155;
  --ink3:        #4A4F72;
  --ink4:        #7A7F9A;
  --code-fg:     #A8D8FF;
  --accent:      #1B2A6B;
  --accent-dk:   #111C4E;
  --accent-lt:   #3D52A0;
  --accent-bg:   #EEF0FA;
  --accent-bdr:  #C4C9E8;
  --green:       #059669;
  --green-bg:    #f0fdf4;
  --amber:       #D97706;
  --amber-bg:    #fffbeb;
  --red:         #DC2626;
  --red-bg:      #fef2f2;
  --blue:        #2563EB;
  --blue-bg:     #eff6ff;
  --border:      rgba(0,0,0,0.1);
  --border2:     rgba(0,0,0,0.06);
  --max:         1080px;
  --max-article: 760px;
  --r:           8px;
  --r-lg:        12px;
  --t:           0.15s ease;
  --nav-h:       60px;
  --chap-nav-h:  44px;
}

/* ---------- RESET ---------- */
*,*::before,*::after{box-sizing:border-box;margin:0;padding:0;}
html{scroll-behavior:smooth;-webkit-text-size-adjust:100%;}
body{
  font-family:-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif;
  background:var(--bg);
  color:var(--ink2);
  font-size:16px;
  line-height:1.65;
  overflow-x:hidden;
}
img{max-width:100%;display:block;}
a{color:var(--accent);}
ul,ol{list-style:none;}
button{font-family:inherit;cursor:pointer;}

/* ---------- ACCESSIBILITY ---------- */
.skip-link{
  position:absolute;left:-999px;top:0;z-index:1000;
  background:var(--accent);color:#fff;padding:10px 18px;border-radius:0 0 var(--r) 0;
  font-size:14px;font-weight:600;text-decoration:none;
}
.skip-link:focus{left:0;}
:focus-visible{outline:3px solid var(--accent-lt);outline-offset:2px;}
.sr-only{position:absolute;width:1px;height:1px;overflow:hidden;clip:rect(0,0,0,0);}

/* ---------- BACK LINK ---------- */
.back-link{
  display:block;background:var(--accent-dk);color:#dfe3f7;
  font-size:13px;text-decoration:none;text-align:center;padding:7px 12px;
}
.back-link:hover{color:#fff;}

/* ---------- TOP NAV ---------- */
.top-nav{
  position:sticky;top:0;z-index:100;height:var(--nav-h);
  background:rgba(247,248,252,0.96);backdrop-filter:blur(10px);
  border-bottom:1px solid var(--border);
  display:flex;align-items:center;gap:14px;padding:0 24px;
}
.logo-mark{
  width:34px;height:34px;border-radius:7px;background:var(--accent);
  color:#fff;font-weight:700;font-size:14px;
  display:flex;align-items:center;justify-content:center;flex-shrink:0;
}
.nav-name{font-weight:600;color:var(--ink);font-size:15px;text-decoration:none;white-space:nowrap;}
.nav-links{display:flex;gap:2px;margin-left:auto;}
.nav-link{font-size:13.5px;color:var(--ink3);text-decoration:none;padding:8px 12px;border-radius:var(--r);transition:background var(--t),color var(--t);}
.nav-link:hover{background:var(--accent-bg);color:var(--accent);}
.nav-search{
  border:1px solid var(--border);border-radius:var(--r);padding:7px 12px;
  font-size:13.5px;background:var(--white);color:var(--ink3);width:180px;
}
.hamburger{display:none;background:none;border:none;padding:8px;flex-direction:column;gap:5px;margin-left:auto;}
.hamburger span{display:block;width:22px;height:2px;background:var(--ink3);border-radius:2px;}
.mobile-drawer{display:none;position:fixed;inset:0;z-index:99;background:var(--bg);padding:76px 24px 24px;flex-direction:column;overflow-y:auto;}
.mobile-drawer.open{display:flex;}
.mobile-drawer a{font-size:16px;color:var(--ink2);text-decoration:none;padding:14px 0;border-bottom:1px solid var(--border);}

/* ---------- CHAPTER NAV ---------- */
.chapter-nav{
  position:sticky;top:var(--nav-h);z-index:90;height:var(--chap-nav-h);
  background:var(--white);border-bottom:1px solid var(--border2);
  display:flex;align-items:center;gap:4px;padding:0 24px;overflow-x:auto;
  white-space:nowrap;
}
.chapter-nav a{font-size:12.5px;color:var(--ink4);text-decoration:none;padding:6px 10px;border-radius:var(--r);}
.chapter-nav a:hover,.chapter-nav a.active{color:var(--accent);background:var(--accent-bg);}

/* ---------- HERO ---------- */
.hero{
  background:linear-gradient(135deg,#0D0F2A 0%,#1B2A6B 60%,#0D1A3E 100%);
  color:#fff;padding:56px 24px 44px;
}
.hero-inner{max-width:var(--max);margin:0 auto;}
.breadcrumb{font-size:12.5px;color:#AEB6E0;margin-bottom:18px;}
.breadcrumb a{color:#AEB6E0;text-decoration:none;}
.breadcrumb a:hover{color:#fff;}
.label-tag{
  display:inline-block;font-size:11px;font-weight:600;letter-spacing:0.08em;text-transform:uppercase;
  background:rgba(255,255,255,0.12);color:#C9D0F5;padding:5px 12px;border-radius:20px;margin-bottom:16px;
}
.hero h1{font-size:clamp(28px,4.5vw,44px);font-weight:700;line-height:1.15;letter-spacing:-0.01em;margin-bottom:14px;}
.hero-meta{display:flex;flex-wrap:wrap;gap:16px;font-size:13px;color:#B7BEE8;}

/* ---------- CANONICAL DEFINITION ---------- */
.canonical-def{
  max-width:var(--max);margin:0 auto;padding:28px 24px 0;
}
.canonical-def p{
  font-size:19px;font-weight:500;color:var(--ink);line-height:1.55;
  border-left:4px solid var(--accent);padding-left:18px;
}

/* ---------- READING LEVEL TABS ---------- */
.level-tabs{
  position:sticky;top:calc(var(--nav-h) + var(--chap-nav-h));z-index:80;
  max-width:var(--max);margin:24px auto 0;padding:0 24px;
  display:flex;gap:8px;background:var(--bg);
}
.level-tab{
  font-size:13.5px;font-weight:600;padding:10px 18px;border-radius:var(--r-lg) var(--r-lg) 0 0;
  border:1px solid var(--border);border-bottom:none;background:var(--bg2);color:var(--ink4);
}
.level-tab[data-level="curious"].active{background:var(--green-bg);color:var(--green);border-color:var(--green);}
.level-tab[data-level="exploring"].active{background:var(--blue-bg);color:var(--blue);border-color:var(--blue);}
.level-tab[data-level="deepdive"].active{background:var(--red-bg);color:var(--red);border-color:var(--red);}
.level-panel{display:none;}
.level-panel.active{display:block;}

/* ---------- TWO-COLUMN LAYOUT ---------- */
.page-body{max-width:var(--max);margin:0 auto;padding:0 24px 60px;}
.content-grid{display:grid;grid-template-columns:65% 1fr;gap:36px;margin-top:20px;}
.article{background:var(--white);border:1px solid var(--border2);border-radius:var(--r-lg);padding:32px;}
.article h2{font-size:22px;color:var(--ink);margin:28px 0 12px;}
.article h2:first-child{margin-top:0;}
.article h3{font-size:17px;color:var(--ink2);margin:20px 0 8px;}
.article p{margin-bottom:14px;max-width:var(--max-article);}
.article ul,.article ol{margin:0 0 14px 20px;list-style:disc;}
.article ol{list-style:decimal;}
.article li{margin-bottom:6px;}
.sidebar{position:sticky;top:calc(var(--nav-h) + var(--chap-nav-h) + 12px);align-self:start;}
.side-box{background:var(--white);border:1px solid var(--border2);border-radius:var(--r-lg);padding:18px;margin-bottom:16px;}
.side-box h4{font-size:12px;font-weight:700;letter-spacing:0.06em;text-transform:uppercase;color:var(--ink4);margin-bottom:10px;}
.side-box a{display:block;font-size:13.5px;color:var(--accent);text-decoration:none;padding:4px 0;}
.side-box a:hover{text-decoration:underline;}

@media (max-width:1024px){
  .content-grid{grid-template-columns:1fr;}
  .sidebar{position:static;}
}

/* ---------- CALLOUT BOXES ---------- */
.callout{border-radius:var(--r);padding:16px 18px;margin:18px 0;border-left:4px solid;font-size:14.5px;}
.callout-title{font-size:11px;font-weight:700;letter-spacing:0.06em;text-transform:uppercase;margin-bottom:6px;display:block;}
.callout-blue{background:var(--blue-bg);border-color:var(--blue);color:#1e3a6e;}
.callout-blue .callout-title{color:var(--blue);}
.callout-green{background:var(--green-bg);border-color:var(--green);color:#164a37;}
.callout-green .callout-title{color:var(--green);}
.callout-amber{background:var(--amber-bg);border-color:var(--amber);color:#7a4a10;}
.callout-amber .callout-title{color:var(--amber);}
.callout-red{background:var(--red-bg);border-color:var(--red);color:#7a1c1c;}
.callout-red .callout-title{color:var(--red);}
.callout-dark{background:var(--bg-code);color:var(--code-fg);border-color:var(--accent-lt);font-family:"Cambria Math",Cambria,serif;overflow-x:auto;}

/* ---------- EQUATION BLOCK ---------- */
.equation-block{
  background:var(--bg-code);color:var(--code-fg);border-radius:var(--r-lg);
  padding:28px;margin:20px 0;text-align:center;overflow-x:auto;font-size:20px;
}
.variable-table{width:100%;border-collapse:collapse;margin:16px 0;font-size:14px;}
.variable-table th,.variable-table td{border:1px solid var(--border);padding:8px 12px;text-align:left;}
.variable-table th{background:var(--bg2);font-weight:600;color:var(--ink2);}

/* ---------- SOURCES ---------- */
.sources{background:var(--bg2);border-radius:var(--r-lg);padding:20px;margin-top:24px;font-size:13.5px;}
.sources h4{font-size:12px;font-weight:700;letter-spacing:0.06em;text-transform:uppercase;color:var(--ink4);margin-bottom:10px;}
.sources li{margin-bottom:8px;color:var(--ink3);}
.sources a{color:var(--accent);}
.page-stamp{font-size:12px;color:var(--ink4);margin-top:12px;padding-top:12px;border-top:1px solid var(--border2);}

/* ---------- RELATED STRIP ---------- */
.related-strip{max-width:var(--max);margin:32px auto 0;padding:0 24px;}
.related-strip h3{font-size:16px;color:var(--ink);margin-bottom:14px;}
.related-grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(220px,1fr));gap:12px;}
.related-card{
  display:block;background:var(--white);border:1px solid var(--border2);border-radius:var(--r);
  padding:14px 16px;text-decoration:none;transition:transform var(--t),box-shadow var(--t);
}
.related-card:hover{transform:translateY(-2px);box-shadow:0 6px 18px rgba(0,0,0,0.07);}
.related-card .rc-label{font-size:10px;font-weight:600;letter-spacing:0.06em;text-transform:uppercase;color:var(--accent);margin-bottom:4px;}
.related-card .rc-title{font-size:14px;color:var(--ink);font-weight:600;}

/* ---------- DOMAIN HUB / CARDS ---------- */
.hub-hero{text-align:left;}
.hub-hero p{max-width:640px;color:#C9D0F5;font-size:15.5px;margin-top:10px;}
.domain-grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(240px,1fr));gap:16px;margin:24px 0;}
.domain-card{
  display:block;background:var(--white);border:1px solid var(--border2);border-radius:var(--r-lg);
  padding:22px;text-decoration:none;transition:transform var(--t),box-shadow var(--t);
}
.domain-card:hover{transform:translateY(-3px);box-shadow:0 10px 26px rgba(27,42,107,0.1);}
.domain-card .dc-icon{
  width:36px;height:36px;border-radius:8px;background:var(--accent-bg);color:var(--accent);
  display:flex;align-items:center;justify-content:center;font-weight:700;margin-bottom:12px;
}
.domain-card h3{font-size:16px;color:var(--ink);margin-bottom:6px;}
.domain-card p{font-size:13px;color:var(--ink3);margin-bottom:8px;}
.domain-card .dc-count{font-size:11.5px;color:var(--ink4);font-weight:600;}

.section-hub-grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(200px,1fr));gap:14px;margin:24px 0;}

/* ---------- STATS BAR (homepage) ---------- */
.stats-bar{display:flex;flex-wrap:wrap;gap:0;border:1px solid rgba(255,255,255,0.18);border-radius:var(--r-lg);overflow:hidden;margin-top:24px;background:rgba(255,255,255,0.05);}
.stat{padding:16px 26px;border-right:1px solid rgba(255,255,255,0.15);text-align:center;flex:1;min-width:120px;}
.stat:last-child{border-right:none;}
.stat-num{display:block;font-size:26px;font-weight:700;color:#fff;line-height:1;margin-bottom:4px;}
.stat-label{font-size:10.5px;letter-spacing:0.07em;text-transform:uppercase;color:#B7BEE8;}

/* ---------- SECTION HEAD ---------- */
.sec-head{display:flex;align-items:center;gap:12px;margin:40px 0 8px;max-width:var(--max);margin-left:auto;margin-right:auto;padding:0 24px;}
.sec-head h2{font-size:20px;color:var(--ink);white-space:nowrap;}
.sec-line{flex:1;height:1px;background:var(--border);}

/* ---------- FOOTER ---------- */
footer{background:var(--accent-dk);color:#C9D0F5;margin-top:60px;}
.ft{max-width:var(--max);margin:0 auto;padding:44px 24px 24px;}
.ft-cols{display:grid;grid-template-columns:2fr 1fr 1fr 1fr;gap:32px;margin-bottom:28px;}
.ft-brand{font-size:17px;font-weight:700;color:#fff;margin-bottom:8px;}
.ft-desc{font-size:13px;line-height:1.6;color:#9CA6D9;}
.ft-ct{font-size:11px;font-weight:700;letter-spacing:0.06em;text-transform:uppercase;color:#7C87BE;margin-bottom:10px;}
.ft-col a{display:block;font-size:13.5px;color:#C9D0F5;text-decoration:none;padding:4px 0;}
.ft-col a:hover{color:#fff;}
.ft-bottom{border-top:1px solid rgba(255,255,255,0.12);padding-top:18px;display:flex;flex-wrap:wrap;justify-content:space-between;gap:12px;align-items:center;}
.ft-disc{font-size:12px;color:#8A93C4;max-width:600px;line-height:1.6;}
.ft-disc a{color:#AEB6E0;}
.ft-copy{font-size:12px;color:#7C87BE;white-space:nowrap;}

/* ---------- HOMEPAGE INTRO ---------- */
.intro-block{max-width:var(--max);margin:0 auto;padding:36px 24px 0;}
.intro-block p{font-size:15.5px;color:var(--ink3);max-width:700px;line-height:1.75;}

/* ---------- RESPONSIVE ---------- */
@media (max-width:1024px){
  .ft-cols{grid-template-columns:1fr 1fr;}
}
@media (max-width:768px){
  .nav-links,.nav-search{display:none;}
  .hamburger{display:flex;}
  .ft-cols{grid-template-columns:1fr 1fr;gap:24px;}
  .hero{padding:40px 20px 32px;}
  .article{padding:22px;}
}
@media (max-width:375px){
  .hero h1{font-size:26px;}
  .stat{padding:12px 16px;}
  .ft-cols{grid-template-columns:1fr;}
}

/* Tap targets */
a,button,.nav-link,.chapter-nav a,.level-tab{min-height:44px;display:inline-flex;align-items:center;}
.domain-card,.related-card,.side-box a{min-height:auto;}

/* ---------- SIMULATION / CALCULATOR SHELL ---------- */
.sim-shell{background:var(--bg-code);border-radius:var(--r-lg);padding:20px;margin:20px 0;color:#fff;}
.sim-controls{display:flex;flex-wrap:wrap;gap:14px;margin-bottom:14px;}
.sim-controls label{font-size:12.5px;color:var(--code-fg);display:flex;flex-direction:column;gap:4px;}
.sim-controls input[type="range"]{width:160px;}
.sim-canvas{width:100%;background:#000;border-radius:var(--r);display:block;}
.sim-readouts{display:grid;grid-template-columns:repeat(auto-fit,minmax(120px,1fr));gap:10px;margin-top:14px;font-size:13px;}
.sim-readouts div{background:rgba(255,255,255,0.06);border-radius:var(--r);padding:8px 12px;}
.sim-btn{background:var(--accent-lt);color:#fff;border:none;border-radius:var(--r);padding:8px 16px;font-size:13px;font-weight:600;margin-right:8px;}
.sim-btn:hover{background:var(--accent);}