refactor(app): carve the highway string-colours out of app.js (R3a) (#883)

static/js/highway-colors.js (601 lines) — bodies VERBATIM.
app.js 10,415 → 9,837. Under 10k.

DECOMPOSED, not sliced. The "settings" blob measured 47 fns / 19 inbound and was
not carvable as-is. Seeding the closure from a FUNCTION (initHighwayColors) found
only 18 fns and left 4 HWC_* constants used outside it — i.e. the seed was wrong,
not the cluster. Re-seeding from the STATE (every function touching HWC_*/`_hwc*`)
found the true cluster: 45 top-level nodes, lines 2751-3331, CONTIGUOUS, with ZERO
foreign nodes inside the span.

  INBOUND: 0.  EXPORTS: 2 (initHighwayColors, hwcInitSettingsUI).

The other 43 symbols — the HWC_* tables, the 12 presets, the theme store, the
share codec, the picker handlers, the window.feedBack.highwayColors facade — are
used nowhere else in core and stay private. No inline on*= handlers here (the
Settings buttons are wired by addEventListener inside hwcInitSettingsUI), so
nothing needed re-exposing on window. The three bus listeners register inside
initHighwayColors, which app.js calls — not at module top level — so no ordering
change.

THE no-undef GATE EARNED ITS KEEP. My closure said INBOUND=0; the module actually
uses `uiPrompt` (the "name this theme" prompt). It was missed because uiPrompt is
no longer an app.js DECLARATION — it's an IMPORT BINDING (from #882's dom.js), and
I was collecting declarations only. `no-undef` with typeof:true caught it.
  => Lesson for the next carve: seed `tops` from ImportDeclaration bindings too.
  => And it VALIDATES carving dom.js early: this module just imports uiPrompt from
     it. Had dom.js still been stranded in app.js, this carve would have needed a
     host seam.

  app.js -> { plugin-loader, viz, diagnostics-export, dom, highway-colors }
  plugin-loader -> viz
  highway-colors -> dom
  viz, diagnostics-export, dom -> (leaves)

VERIFIED BY DRIVING THE FACADE. A/B against origin/main in two browsers:
window.feedBack.highwayColors installed, identical method surface, 12 presets,
identical default slot colours, and a share-code encode→decode round-trip
returning #112233 — IDENTICAL on both, zero console/page errors either side.

Harnesses: highway_colors_facade + highway_string_colors retargeted (both
brace-extract blocks out of the source by signature).

pytest 2396, node 1038/1038, ESLint 0, tailwind-fresh clean, Codex 0.

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

tests/js/highway_colors_facade.test.js

@@ -8,7 +8,9 @@ const assert = require('node:assert/strict');
 const fs = require('node:fs');
 const path = require('node:path');
 
-const appJs = path.join(__dirname, '..', '..', 'static', 'app.js');
+// The highway string-colour manager was carved out of app.js into its own
+// module (R3a).
+const appJs = path.join(__dirname, '..', '..', 'static', 'js', 'highway-colors.js');
 
 function extractBlock(src, signature) {
     const start = src.indexOf(signature);

tests/js/highway_string_colors.test.js

@@ -13,7 +13,9 @@ const path = require('node:path');
 
 const highwayJs = path.join(__dirname, '..', '..', 'static', 'highway.js');
 const highway3dJs = path.join(__dirname, '..', '..', 'plugins', 'highway_3d', 'screen.js');
-const appJs = path.join(__dirname, '..', '..', 'static', 'app.js');
+// The highway string-colour manager was carved out of app.js into its own
+// module (R3a).
+const appJs = path.join(__dirname, '..', '..', 'static', 'js', 'highway-colors.js');
 
 // Brace-balanced extraction (same helper shape as highway_note_state.test.js).
 function extractBlock(src, signature) {
@@ -86,7 +88,7 @@ test('3D gem-body gradients follow the active palette (not hardcoded)', () => {
     assert.match(apply, /_recolorGemGradients\(\)/, '_applyPaletteToMaterials must recolor gems on palette change');
 });
 
-// ── Core color manager (static/app.js) ────────────────────────────────────
+// ── Core color manager (static/js/highway-colors.js) ──────────────────────
 
 test('app.js color manager name-maps to both highways, with identity no-op + builtin guard', () => {
     const src = fs.readFileSync(appJs, 'utf8');