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_teaching_marks.test.js

@@ -32,7 +32,7 @@ const fingerLabel2D = loadFn('static/js/highway-geometry.js', 'teachingFingerLab
 const degreeLabel2D = loadFn('static/js/highway-geometry.js', 'teachingDegreeLabel');
 const fingerLabel3D = loadFn('plugins/highway_3d/screen.js', 'teachingFingerLabel');
 const degreeLabel3D = loadFn('plugins/highway_3d/screen.js', 'teachingDegreeLabel');
-const strumGroupBuckets = loadFn('static/highway.js', 'strumGroupBuckets');
+const strumGroupBuckets = loadFn('static/js/highway-draw.js', 'strumGroupBuckets');
 
 // ── teachingFingerLabel (fg) ─────────────────────────────────────────────────