refactor(h3d-carve-6): move N-section material builders to src/materials.js

Extract TXT_STYLES + 13 material builder functions (txtMat, pinchHarmonicMat,
naturalHarmonicMat, palmMuteXSpriteMat, fretHandMuteXSpriteMat, muteXMat,
triMat, bendChevronMat, darkenHex, slideArrowMat, _meshMatForGhostFretDigit,
_spriteMat2MeshMat) and pool() from screen.js N-section into:
  plugins/highway_3d/src/materials.js  (createMaterialBuilders factory)

screen.js drops ~600 lines (15302→14705).

Surprises vs plan §4:
  • DI is 4 params { getT, getTxtCache, techMatCache, techMeshMatClones } not 1
  • _syncOpenStringPitchLabels cluster excluded (20+ factory-scope deps)
  • _techMatCache stays in screen.js factory scope for teardown .values()/.clear()

Beyond-subst (4):
  1. Factory wrapper createMaterialBuilders({...})
  2. T → const T = getT() inside each function body (live accessor)
  3. txtCache[k] → const cache = getTxtCache(); cache[k]
  4. _techMatCache/_techMeshMatClones → DI param names techMatCache/techMeshMatClones

Tests: 16 new class-killer tests in highway_3d_materials.test.js;
pool warm tests retargeted to src/materials.js; panel_controls stub added.
Full suite: 1246/1248 pass (2 pre-existing: network + nut-labels).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014uZ169yfoFYArXz962g7KW

tests/js/highway_3d_panel_controls.test.js

@@ -104,6 +104,14 @@ function loadHighway3dStatics() {
         createBgControl: () => ({
             _pcAcquire() {}, _pcRelease() {},
         }),
+        // h3d-carve-6: material builders moved to src/materials.js.
+        createMaterialBuilders: () => ({
+            txtMat() {}, pinchHarmonicMat() {}, naturalHarmonicMat() {},
+            palmMuteXSpriteMat() {}, fretHandMuteXSpriteMat() {}, muteXMat() {},
+            triMat() {}, bendChevronMat() {}, darkenHex: (hex) => hex, slideArrowMat() {},
+            _meshMatForGhostFretDigit() {}, _spriteMat2MeshMat() {},
+            pool: () => ({ get() {}, reset() {}, warm() { return this; } }),
+        }),
     };
     vm.createContext(sandbox);
     vm.runInContext(instrumented, sandbox, { filename: SCREEN_JS });