/* Layout idea: an engineer's computation pad. Green-ruled paper, a drawing title block for the facts,
   and one object on a green drafting-board cover: the rule itself, drawn full width. */
:root{
  color-scheme:light;
  --paper:#eaf0de; --grid:#d5e0c6; --panel:#f6f9ee; --ink:#18261f; --ink-2:#4a5b51; --line:#aebf9e;
  --red:#a8261c; --board:#2e4a3e; --board-2:#223a30; --on-board:#e9f0e2; --on-board-2:#b7c9bb;
  --mark:#ffd23f;
  --f-display:"Barlow Condensed","Arial Narrow",sans-serif;
  --f-ui:"Barlow","Helvetica Neue",Arial,sans-serif;
  --f-body:"Besley",Georgia,"Times New Roman",serif;
}
*{box-sizing:border-box}
html{-webkit-text-size-adjust:100%}
body{margin:0;background:var(--paper);color:var(--ink);font-family:var(--f-body);font-size:17px;line-height:1.6;
  background-image:linear-gradient(var(--grid) 1px,transparent 1px),linear-gradient(90deg,var(--grid) 1px,transparent 1px);
  background-size:22px 22px;overflow-x:hidden}
.wrap{max-width:1180px;margin:0 auto;padding:0 20px}
h1,h2,h3{font-family:var(--f-display);font-weight:600;text-wrap:balance;margin:0;line-height:1.02;letter-spacing:.01em}
h1{font-size:clamp(2.9rem,8vw,5.4rem);text-transform:uppercase;font-weight:700}
h1 em{font-style:normal;color:var(--red)}
h2{font-size:clamp(1.8rem,4vw,2.5rem);text-transform:uppercase}
h3{font-size:1.25rem;text-transform:uppercase;letter-spacing:.06em}
p{margin:0}
a{color:inherit;text-decoration-color:var(--red);text-underline-offset:3px}
button,input{font:inherit}
:focus-visible{outline:3px solid var(--mark);outline-offset:2px}

.head{padding-top:36px;padding-bottom:26px;display:grid;gap:18px}
.eyebrow{font-family:var(--f-display);font-weight:600;letter-spacing:.2em;text-transform:uppercase;font-size:.95rem;color:var(--ink-2)}
.lede{font-size:1.12rem}
.titleblock{display:grid;grid-template-columns:repeat(5,auto);margin:6px 0 0;border:1.5px solid var(--ink);background:var(--panel);width:fit-content;max-width:100%}
.titleblock div{padding:7px 16px 8px;border-right:1px solid var(--ink)}
.titleblock div:last-child{border-right:0}
.titleblock dt{font-family:var(--f-display);font-size:.72rem;letter-spacing:.18em;text-transform:uppercase;color:var(--ink-2)}
.titleblock dd{margin:0;font-family:var(--f-display);font-weight:600;font-size:1.22rem;line-height:1.15;font-variant-numeric:tabular-nums}
@media (max-width:760px){.titleblock{grid-template-columns:1fr 1fr;width:100%}.titleblock div{border-bottom:1px solid var(--ink)}.titleblock div:nth-child(2n){border-right:0}.titleblock div:last-child{grid-column:1/-1;border-bottom:0}}

/* ---------- stage ---------- */
.stage{background:var(--board);background-image:radial-gradient(120% 90% at 50% 0%,transparent 40%,var(--board-2));color:var(--on-board);padding:26px 0 26px;border-block:1.5px solid var(--ink);overflow-x:clip}
#rule{display:block;width:100%;height:auto;overflow:visible;touch-action:pan-y;user-select:none;-webkit-user-select:none}
#rule.flipping{animation:flip .46s ease-in-out}
@keyframes flip{50%{transform:scaleY(.03)}}
[data-drag]{cursor:grab}
.dragging [data-drag]{cursor:grabbing}
.stock{fill:url(#ivory);stroke:#b3a783;stroke-width:.6}
.slidebody{fill:url(#ivory2);stroke:#a99c76;stroke-width:.7}
.shadow{fill:#000;opacity:.28}
.tk{stroke:#1b1a17;fill:none;stroke-width:.55}
.tk.r2,.tk.r3{stroke-width:.7}
.base{stroke:#1b1a17;stroke-width:.45;fill:none}
.lb{font:500 7.6px var(--f-ui);fill:#1b1a17;text-anchor:middle}
.lb.s{font-size:5.4px}
.lb.r{fill:#a8261c}
.lb.i{font-style:italic;font-size:6.2px}
.nm{font:600 8.6px var(--f-ui);fill:#1b1a17;text-anchor:end;letter-spacing:.06em}
.nm.r{fill:#a8261c}
.mk{font:600 5px var(--f-ui);fill:#a8261c;letter-spacing:.09em}
.tab{font:600 6px var(--f-ui);fill:#3a372f;text-anchor:middle;letter-spacing:.05em}
.hand{font:600 19px "Caveat","Segoe Script",cursive;fill:#2b2f33;opacity:.82}
.hl{fill:var(--mark);opacity:0;transition:opacity .25s}
.hl.on{opacity:.42}
.glass{fill:#dff3ee;fill-opacity:.16;stroke:#8d9494;stroke-width:1.2}
.rail{fill:url(#metal);stroke:#6b7070;stroke-width:.6}
.hair{stroke:#c0281c;stroke-width:.9}
#loupe .band{overflow:hidden}
.ln-bg{fill:#fbf7e8;stroke:#1b1a17;stroke-width:.5}
.ln-bg.on{fill:var(--mark)}
.ln-bar{fill:#b3a783}.ln-bar.sl{fill:#5f7f70}
.ln-t{font:600 8.4px var(--f-ui);fill:#1b1a17;letter-spacing:.04em}
.ln-t.r{fill:#a8261c}
.ln-v{text-anchor:end;letter-spacing:0;font-variant-numeric:tabular-nums}

.caption{margin-top:16px;display:grid;grid-template-columns:1fr auto;gap:14px 22px;align-items:center;min-height:74px}
#cap{font-size:1.06rem;max-width:70ch}
#cap b{font-family:var(--f-ui);font-weight:600;color:#fff;background:rgba(255,210,63,.22);padding:0 .3em;border-radius:3px}
#stepno{font-family:var(--f-display);font-weight:600;letter-spacing:.16em;text-transform:uppercase;font-size:1rem;color:var(--mark);display:block;margin-bottom:2px}
.stepnav{display:flex;gap:8px;flex-wrap:wrap}
.btn{font-family:var(--f-display);font-weight:600;font-size:1.05rem;letter-spacing:.08em;text-transform:uppercase;padding:8px 16px 9px;border:1.5px solid var(--ink);background:var(--panel);color:var(--ink);border-radius:3px;cursor:pointer;line-height:1.1}
.btn:hover{background:var(--mark);color:#18261f}
.btn.primary{background:var(--red);color:#fff;border-color:var(--red)}
.btn.primary:hover{filter:brightness(1.12);background:var(--red);color:#fff}
.stage .btn{border-color:var(--on-board);background:transparent;color:var(--on-board)}
.stage .btn:hover{background:var(--on-board);color:#18261f}
.stage .btn.primary{background:var(--mark);border-color:var(--mark);color:#18261f}
.btn[disabled]{opacity:.35;cursor:default;pointer-events:none}

.bench{margin-top:20px;display:grid;grid-template-columns:minmax(0,1.15fr) minmax(0,1fr);gap:22px;align-items:start}
.loupebox{position:relative}
#loupe{display:block;width:100%;aspect-ratio:25/17.8;border-radius:14px;border:3px solid #9aa3a0;background:var(--board-2);box-shadow:inset 0 0 0 1px #0006, 0 6px 0 #0003}
.readhead{font-family:var(--f-display);font-weight:600;letter-spacing:.16em;text-transform:uppercase;font-size:.82rem;color:var(--on-board-2)}
.readhead{display:flex;justify-content:space-between;gap:10px;align-items:baseline;margin-bottom:8px;flex-wrap:wrap}
.chips{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));grid-template-rows:repeat(5,auto);grid-auto-flow:column;gap:6px}
.chip{display:flex;justify-content:space-between;align-items:baseline;gap:8px;padding:5px 10px 6px;background:#f3ecd6;color:#1b1a17;border:0;border-left:5px solid #b3a783;border-radius:2px;cursor:text;font-family:var(--f-ui);text-align:left;min-width:0}
.chip.sl{border-left-color:#5f7f70}
.chip .n{font-weight:600;letter-spacing:.05em;font-size:.92rem}
.chip .n.r{color:#a8261c}
.chip .v{flex:1;width:100%;min-width:0;padding:0;border:0;background:transparent;color:inherit;font-weight:500;font-size:1rem;font-variant-numeric:tabular-nums;text-align:right}
.chip .v:focus-visible{outline:0}
.chip:focus-within{outline:3px solid var(--mark);outline-offset:1px}
.chip.bad{background:#f2b8ad}
.chip.on{background:var(--mark)}
.chip.off .v{opacity:.45;font-size:.82rem}
.legend{margin-top:8px;font-family:var(--f-ui);font-size:.82rem;color:var(--on-board-2);display:flex;gap:16px;flex-wrap:wrap}
.legend i{display:inline-block;width:5px;height:12px;vertical-align:-1px;margin-right:6px;background:#b3a783}
.legend i.sl{background:#5f7f70}
.controls{margin-top:14px;display:grid;gap:10px}
.controls .btns{display:flex;gap:8px;flex-wrap:wrap}
@media (max-width:820px){.bench{grid-template-columns:1fr}.caption{grid-template-columns:1fr}}

/* ---------- lessons ---------- */
.lessons{display:grid;grid-template-columns:250px minmax(0,1fr);gap:34px;align-items:start}
.lnav{display:grid;gap:0;border:1.5px solid var(--ink);background:var(--panel)}
.lnav button{display:grid;grid-template-columns:30px 1fr;gap:6px;align-items:baseline;text-align:left;padding:9px 12px;border:0;border-bottom:1px solid var(--line);background:transparent;color:var(--ink);cursor:pointer;font-family:var(--f-display);font-weight:600;font-size:1.12rem;letter-spacing:.03em;line-height:1.15}
.lnav button:last-child{border-bottom:0}
.lnav button .k{color:var(--ink-2);font-variant-numeric:tabular-nums}
.lnav button[aria-current="true"]{background:var(--ink);color:var(--paper)}
.lnav button[aria-current="true"] .k{color:var(--mark)}
.lnav button:hover:not([aria-current="true"]){background:var(--grid)}
.lesson{display:grid;gap:16px;min-height:360px;align-content:start}
.lesson p{max-width:64ch}
.lesson .kick{font-family:var(--f-display);font-weight:600;letter-spacing:.2em;text-transform:uppercase;font-size:.9rem;color:var(--red)}
.lesson .acts{display:flex;gap:10px;flex-wrap:wrap;align-items:end}
.field{display:grid;gap:2px;font-family:var(--f-display);font-weight:600;letter-spacing:.12em;text-transform:uppercase;font-size:.8rem;color:var(--ink-2)}
.field input[type=number]{width:110px;padding:7px 9px;border:1.5px solid var(--ink);border-radius:3px;background:var(--panel);color:var(--ink);font-family:var(--f-ui);font-weight:600;font-size:1.1rem;letter-spacing:0}
.op{font-family:var(--f-display);font-weight:600;font-size:1.7rem;padding-bottom:4px}
.msg{font-family:var(--f-ui);font-size:.92rem;color:var(--red);min-height:1.2em}
.demo{border:1.5px solid var(--ink);background:var(--panel);padding:14px 16px 12px;display:grid;gap:8px;max-width:680px}
.demo svg{display:block;width:100%;height:auto}
.demo .eq{font-family:var(--f-display);font-weight:600;font-size:1.7rem;line-height:1.1;font-variant-numeric:tabular-nums}
.demo .eq small{display:block;font-family:var(--f-ui);font-weight:500;font-size:.9rem;color:var(--ink-2);letter-spacing:0;margin-top:2px}
.demo .sl{display:grid;grid-template-columns:86px 1fr;gap:10px;align-items:center;font-family:var(--f-display);font-weight:600;letter-spacing:.1em;text-transform:uppercase;font-size:.85rem}
.demo input[type=range]{width:100%}
.demo .ca{accent-color:#2f6f8f}.demo .cb{accent-color:#b5521b}
.d-rul{fill:#f3ecd6;stroke:#8f8566;stroke-width:1}
.d-tk{stroke:#1b1a17;stroke-width:1}
.d-lb{font:600 11px var(--f-ui);fill:#1b1a17;text-anchor:middle}
.d-a{fill:#2f6f8f}.d-b{fill:#b5521b}
.d-at{font:600 12px var(--f-ui);fill:#2f6f8f;text-anchor:middle}
.d-bt{font:600 12px var(--f-ui);fill:#b5521b;text-anchor:middle}
.d-ln{stroke:var(--red);stroke-width:1.4;stroke-dasharray:4 3}
.d-res{font:700 15px var(--f-ui);fill:var(--red);text-anchor:middle}
@media (max-width:820px){.lessons{grid-template-columns:1fr;gap:20px}.lnav{grid-auto-flow:column;grid-auto-columns:max-content;overflow-x:auto}.lnav button{border-bottom:0;border-right:1px solid var(--line);grid-template-columns:auto auto}}

.eqrow{display:flex;gap:10px;flex-wrap:wrap;align-items:end}
.eqrow .field{flex:1 1 280px;max-width:560px}
.field input[type=text]{width:100%;padding:8px 11px;border:1.5px solid var(--ink);border-radius:3px;background:var(--panel);color:var(--ink);font-family:var(--f-ui);font-weight:600;font-size:1.2rem;letter-spacing:.02em}
.examples{display:flex;gap:6px;flex-wrap:wrap;align-items:center;font-family:var(--f-display);font-weight:600;letter-spacing:.14em;text-transform:uppercase;font-size:.82rem;color:var(--ink-2)}
.examples button{font-family:var(--f-ui);font-weight:600;font-size:.92rem;letter-spacing:0;text-transform:none;padding:3px 10px 4px;border:1px solid var(--line);border-radius:20px;background:var(--panel);color:var(--ink);cursor:pointer}
.examples button:hover{border-color:var(--ink);background:var(--mark);color:#18261f}
.help{font-family:var(--f-ui);font-size:.95rem;max-width:680px}
.help summary{cursor:pointer;font-family:var(--f-display);font-weight:600;letter-spacing:.12em;text-transform:uppercase;font-size:.88rem;color:var(--ink-2)}
.help ul{margin:8px 0 0;padding-left:20px;display:grid;gap:4px}
.help code,.eq-ans code{font-family:var(--f-ui);font-weight:600;background:var(--grid);padding:0 .3em;border-radius:3px}
.eqout{display:grid;gap:10px;max-width:820px}
.eq-ans{font-family:var(--f-display);font-weight:600;font-size:1.7rem;line-height:1.15;font-variant-numeric:tabular-nums}
.eq-ans small{display:block;font-family:var(--f-ui);font-weight:500;font-size:.9rem;color:var(--ink-2)}
.eqsteps{list-style:none;margin:0;padding:0;border:1.5px solid var(--ink);background:var(--panel);counter-reset:s}
.eqsteps li{counter-increment:s;border-bottom:1px solid var(--line)}
.eqsteps li:last-child{border-bottom:0}
.eqsteps button{display:grid;grid-template-columns:30px 1fr auto;gap:8px;align-items:baseline;width:100%;text-align:left;padding:8px 12px;border:0;background:transparent;color:var(--ink);cursor:pointer;font-family:var(--f-ui);font-size:.97rem;line-height:1.4}
.eqsteps li.part button{cursor:default;color:var(--ink)}
.eqsteps li.part button::before{content:none}
.eqsteps button::before{content:counter(s);font-family:var(--f-display);font-weight:600;font-size:1.05rem;color:var(--ink-2)}
.eqsteps button:hover{background:var(--grid)}
.eqsteps li.cur button{background:var(--mark);color:#18261f}
.eqsteps li.cur button::before{color:#18261f}
.eqsteps .sd{font-family:var(--f-display);font-weight:600;letter-spacing:.12em;text-transform:uppercase;font-size:.72rem;color:var(--ink-2);white-space:nowrap}
.eqsteps li.cur .sd{color:#18261f}
.eqsteps li.part button{background:var(--grid);font-weight:600}

/* ---------- reference + photos ---------- */
.sec{padding-top:46px;display:grid;gap:16px}
.sec>p{max-width:64ch}
.tablewrap{overflow-x:auto;border:1.5px solid var(--ink);background:var(--panel)}
table{border-collapse:collapse;width:100%;min-width:560px;font-family:var(--f-ui);font-size:.95rem}
th,td{text-align:left;padding:7px 12px;border-bottom:1px solid var(--line);vertical-align:baseline}
tr:last-child td{border-bottom:0}
th{font-family:var(--f-display);letter-spacing:.14em;text-transform:uppercase;font-size:.8rem;color:var(--ink-2);border-bottom:1.5px solid var(--ink)}
td.n{font-weight:600;white-space:nowrap}
td.n.r{color:var(--red)}
td.w{white-space:nowrap;color:var(--ink-2)}
table.mini{min-width:0;max-width:560px;border:1.5px solid var(--ink);background:var(--panel)}
table.mini td.w{color:var(--ink);font-weight:600;text-align:right}
.photos{display:grid;gap:18px}
figure{margin:0;display:grid;gap:6px}
figure img{display:block;width:100%;max-width:100%;height:auto;border:1.5px solid var(--ink)}
figcaption{font-size:.95rem;color:var(--ink-2);max-width:70ch}
footer{padding:40px 0 60px;font-size:.9rem;color:var(--ink-2)}
footer p{max-width:70ch}
@media (prefers-reduced-motion: reduce){#rule.flipping{animation:none}.hl{transition:none}}

/* ---------- site chrome ---------- */
.sitenav{background:var(--panel);border-bottom:1.5px solid var(--ink)}
.sitenav .wrap{display:flex;justify-content:space-between;align-items:center;gap:16px;min-height:54px}
.brand{font-family:var(--f-display);font-weight:700;font-size:1.2rem;letter-spacing:.08em;text-transform:uppercase;text-decoration:none}
.navlinks{display:flex;gap:22px}
.navlinks a{font-family:var(--f-display);font-weight:600;font-size:1.05rem;letter-spacing:.12em;text-transform:uppercase;text-decoration:none;padding:15px 0 12px;border-bottom:3px solid transparent}
.navlinks a:hover{border-bottom-color:var(--line)}
.navlinks a[aria-current="page"]{border-bottom-color:var(--red)}
.sitefoot{border-top:1px solid var(--line);margin-top:30px;padding-top:18px}

/* ---------- tabs ---------- */
[hidden]{display:none!important}
.workbench{padding-top:34px;padding-bottom:20px}
.tabs{display:flex;gap:6px;border-bottom:1.5px solid var(--ink);margin-bottom:26px}
.tabs button{font-family:var(--f-display);font-weight:600;font-size:1.2rem;letter-spacing:.08em;text-transform:uppercase;line-height:1.1;padding:10px 22px 11px;border:1.5px solid var(--ink);border-bottom:0;border-radius:3px 3px 0 0;background:var(--panel);color:var(--ink);cursor:pointer}
.tabs button:hover{background:var(--grid)}
.tabs button[aria-selected="true"]{background:var(--ink);color:var(--paper)}

/* ---------- scale guide ---------- */
dialog.scales{width:min(980px,calc(100vw - 32px));max-height:calc(100dvh - 48px);padding:0;border:1.5px solid var(--ink);border-radius:4px;background:var(--paper);color:var(--ink);overflow-y:auto}
dialog.scales::backdrop{background:rgba(24,38,31,.62)}
.modalhead{position:sticky;top:0;z-index:1;display:flex;justify-content:space-between;align-items:center;gap:16px;padding:14px 20px;background:var(--panel);border-bottom:1.5px solid var(--ink)}
.modalbody{display:grid;gap:14px;padding:18px 20px 22px}
#stepno .eqname{display:inline-block;margin:0 12px 8px 0;padding:3px 14px 4px;background:#f3ecd6;color:#1b1a17;border-left:5px solid var(--mark);border-radius:2px;font-size:1.75rem;font-weight:600;line-height:1.15;letter-spacing:.01em;text-transform:none;font-variant-numeric:tabular-nums;vertical-align:middle;overflow-wrap:anywhere}
.source{font-family:var(--f-ui);font-size:.9rem;color:var(--ink-2);margin-top:-6px}
.caption:has(#stepno:empty):has(#cap:empty){min-height:0}