refactor(highway): carve the PURE geometry primitives into highway-geometry.js (R3c) (#915)

6 functions, 53 lines. highway.js 4,158 -> 4,105. NOT ONE CALL SITE CHANGES.

project, roundRect, bnvNormalizedPoints, teachingFingerLabel, teachingDegreeLabel,
chordHarmonyLabels — the shared primitives every drawing function leans on.

━━━ PURITY IS THE WHOLE POINT OF THIS SLICE ━━━

Every one of these is a pure function of its arguments. None touches hwState. None closes over
the canvas context — roundRect() already took `ctx` explicitly, and the rest need nothing but
numbers. project() reads only the module-level constants from #914.

That matters because createHighway() is a FACTORY: a plugin can build a second highway for its
own panel, so anything holding per-instance state must be PASSED hwState rather than importing
it, or two panels silently share one clock and palette. These six hold no state at all, so
they move VERBATIM — the module boundary is invisible to every caller.

The asserts are mechanical and in the extractor: it REFUSES to move a function whose body
mentions hwState, or that references `ctx` without taking it as a parameter. Purity is
checked, not assumed.

━━━ WHAT IS DELIBERATELY LEFT BEHIND ━━━

The four primitives that DO need hwState — fretX, fillTextReadable, _noteState, _paintGemGlow
— stay in the factory for now. They need an explicit hwState parameter threaded through 53
call sites, which is a real behavioural change and belongs in its own commit rather than
smuggled in beside a provably-identical move. Separating the provable from the risky is the
whole discipline of this epic.

TESTS. Three harnesses brace-match these functions out of the source and run them in a
sandbox; they now read static/js/highway-geometry.js. `export function x` still contains
`function x`, so the extractor needed no change — only the path.

VERIFIED. A/B against origin/main: 15 probes IDENTICAL, zero page errors. PERF GATE PASSES AT
1.91ms against its 12ms budget — and this is the one that could plausibly have cost something:
project() runs for every visible note on every frame and is now a CROSS-MODULE call. It costs
nothing measurable. That is the answer #910 was built to give.

node 1045, pytest 2416, ESLint 0, Codex 0.

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

tests/js/highway_bend_curve.test.js

@@ -25,7 +25,9 @@ function loadFn(file, name) {
     return new Function('"use strict";' + extractFn(src, name) + `\nreturn ${name};`)();
 }
 
-const bnvNormalizedPoints = loadFn('static/highway.js', 'bnvNormalizedPoints');
+// R3c: the PURE geometry/label primitives were carved out of highway.js into
+// static/js/highway-geometry.js. Same bodies, byte-for-byte — only the file moved.
+const bnvNormalizedPoints = loadFn('static/js/highway-geometry.js', 'bnvNormalizedPoints');
 const bnvSampleAt = loadFn('plugins/highway_3d/screen.js', 'bnvSampleAt');
 
 // ── bnvNormalizedPoints (2D) ─────────────────────────────────────────────────

tests/js/highway_chord_harmony.test.js

@@ -25,7 +25,9 @@ function loadFn(file, name) {
     return new Function('"use strict";' + extractFn(src, name) + `\nreturn ${name};`)();
 }
 
-const labels2D = loadFn('static/highway.js', 'chordHarmonyLabels');
+// R3c: the PURE geometry/label primitives were carved out of highway.js into
+// static/js/highway-geometry.js. Same bodies, byte-for-byte — only the file moved.
+const labels2D = loadFn('static/js/highway-geometry.js', 'chordHarmonyLabels');
 const labels3D = loadFn('plugins/highway_3d/screen.js', 'chordHarmonyLabels');
 
 for (const [name, fn] of [['2D', labels2D], ['3D', labels3D]]) {

tests/js/highway_teaching_marks.test.js

@@ -26,8 +26,10 @@ function loadFn(file, name) {
     return new Function('"use strict";' + extractFn(src, name) + `\nreturn ${name};`)();
 }
 
-const fingerLabel2D = loadFn('static/highway.js', 'teachingFingerLabel');
-const degreeLabel2D = loadFn('static/highway.js', 'teachingDegreeLabel');
+// R3c: the PURE geometry/label primitives were carved out of highway.js into
+// static/js/highway-geometry.js. Same bodies, byte-for-byte — only the file moved.
+const fingerLabel2D = loadFn('static/js/highway-geometry.js', 'teachingFingerLabel');
+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');