refactor(highway): carve the 2D drawing layer into highway-draw.js (R3c) (#917)

18 functions, 1,245 lines. highway.js 3,972 -> 2,727 (-31%). The biggest R3c slice: notes,
sustains, chords, strum groups, unison bends and lyrics — everything the default renderer
paints each frame.

━━━ MUTABILITY, NOT LOCATION, DECIDES WHERE A THING BELONGS ━━━

Three per-instance caches came out with this slice, and they are why it needed care:

    _frameMismatchWarned   a warn-once Set of chord ids     (feedBack#88)
    _chordRenderInfo       a WeakMap of chord -> chain info
    _lyricMeasureCache     Map<fontSize, Map<text, width>>

All three are MUTATED. Left at module scope they would be SHARED ACROSS PANELS — one
highway's lyric widths and chord chains stomping another's, silently, with nothing throwing.
createHighway() is a factory (the constitution publishes window.createHighway so a plugin can
build a second highway), so they are lifted onto hwState, which is exactly what hwState is for.

The shimmer LUT went the OTHER way — to MODULE scope in highway-geometry.js. It is a
deterministic xorshift table, byte-for-byte identical for every instance, so sharing it is not
merely safe but BETTER: built once for the page rather than once per panel.

Same slice, opposite directions, decided entirely by whether the thing mutates.

━━━ MY SCRIPT WAS WRONG TWICE. THE GATES CAUGHT BOTH. ━━━

1. HAND-LISTED THE MOVE SET. I listed 10 functions and missed six that drawChords needs
   (_ensureChordRenderCache, bsearchChords, getChordTemplateInfo, _computeChordBox,
   _updateFretLinePreview, _drawFretLineChordPreview). The no-undef gate named every one. The
   set is now DERIVED from the dependency closure — 18, not 10.

2. JUDGED PURITY TOO EARLY, and this one is subtle. I classified _computeChordBox as pure
   because its ORIGINAL body never mentions hwState. Then the call-site rewriter injected
   `fretX(hwState, …)` INTO it — fretX takes hwState now (#916) — leaving a function that
   references an hwState it was never given. Purity has to be judged from the body AS IT WILL
   BE, so the classifier iterates to a fixed point: a function needs hwState if it mentions it,
   OR calls anything that now takes it. That moved _computeChordBox to the stateful side.

VERIFIED. A/B against origin/main: IDENTICAL, zero page errors. The PLUGIN BUNDLE contract is
byte-identical (b.fretX arity 3, b.getNoteState arity 2, both stable references, both correct
under the old calling convention). PERF GATE PASSES AT 1.92ms against its 12ms budget — and
this is the slice that could really have cost something: the ENTIRE per-frame drawing path is
now cross-module. It costs nothing measurable.

TESTS. highway_teaching_marks follows strumGroupBuckets to the new module. The two source-shape
harnesses now read highway.js AND every static/js/highway-*.js, rather than being re-pinned at
whichever file currently holds a function — re-pinning breaks again next time, and a shape
assertion that silently stops finding its target is indistinguishable from one that passes.

node 1045, pytest 2416, ESLint 0, no-undef 0, Codex 0.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/js/highway_chord_render_cache.test.js

@@ -19,8 +19,23 @@ const path = require('node:path');
 
 const highwayJs = path.join(__dirname, '..', '..', 'static', 'highway.js');
 
+
+// R3c: highway.js is being carved into modules, so its source is no longer ONE file. Read the
+// whole set rather than re-pinning at whichever file currently holds a function — re-pinning
+// just breaks again next time, and a source-shape assertion that silently stops finding its
+// target is indistinguishable from one that passes.
+function highwaySources() {
+    const root = path.join(__dirname, '..', '..');
+    const jsDir = path.join(root, 'static', 'js');
+    const parts = [fs.readFileSync(path.join(root, 'static', 'highway.js'), 'utf8')];
+    for (const f of fs.readdirSync(jsDir).sort()) {
+        if (f.startsWith('highway-') && f.endsWith('.js')) parts.push(fs.readFileSync(path.join(jsDir, f), 'utf8'));
+    }
+    return parts.join('\n');
+}
+
 test('_ensureChordRenderCache keys off src, _inverted, AND chordTemplates', () => {
-    const src = fs.readFileSync(highwayJs, 'utf8');
+    const src = highwaySources();
     // The cache key triple must include chordTemplates — without it, a
     // late-arriving `chord_templates` WS message leaves cached
     // nonZeroNotes / nonZeroFrets stale until the next chord transition.
@@ -41,7 +56,7 @@ test('_ensureChordRenderCache keys off src, _inverted, AND chordTemplates', () =
 });
 
 test('chordTemplates change resets fretline preview and frame-mismatch warner', () => {
-    const src = fs.readFileSync(highwayJs, 'utf8');
+    const src = highwaySources();
     // The cache-invalidation block must clear both _chordFretLineNotes
     // (so _updateFretLinePreview re-publishes with corrected isOpen
     // classification) and _frameMismatchWarned (so a chord ID warned