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

plugins/highway_3d/plugin.json

@@ -1,13 +1,20 @@
 {
-    "id": "highway_3d",
-    "name": "3D Highway",
-    "version": "3.41.0",
-    "type": "visualization",
-    "scriptType": "module",
-    "bundled": true,
-    "script": "screen.js",
-    "styles": "assets/plugin.css",
-    "settings": { "html": "settings.html", "category": "graphics", "server_files": ["plugin_uploads/highway_3d/current.mp4", "plugin_uploads/highway_3d/current.webm"] },
-    "routes": "routes.py",
-    "tour": "tour.json"
+  "id": "highway_3d",
+  "name": "3D Highway",
+  "version": "3.42.0",
+  "type": "visualization",
+  "scriptType": "module",
+  "bundled": true,
+  "script": "screen.js",
+  "styles": "assets/plugin.css",
+  "settings": {
+    "html": "settings.html",
+    "category": "graphics",
+    "server_files": [
+      "plugin_uploads/highway_3d/current.mp4",
+      "plugin_uploads/highway_3d/current.webm"
+    ]
+  },
+  "routes": "routes.py",
+  "tour": "tour.json"
 }