refactor(h3d): extract color/tuning/splitscreen utils to src/utils.js (h3d-carve-3)

Moves 12 pure-function / compile-time-constant exports from the screen.js
IIFE into a new src/utils.js ES module:

  Color utils: _h3dHexToInt, _clampByteI, _darkenInt, _lightenInt
  String-count: resolveStringCount
  Tuning/pitch: _NOTE_NAMES_SHARP, _BASE_OPEN_MIDI_BASS4/5, _BASE_OPEN_MIDI_GUITAR6/7/8,
                _baseOpenStringMidis, _midiToPitchLabel, _openStringPitchLabelsForTuning
  Splitscreen:  _ssActive, _ssIsCanvasFocused

Free-identifier audit: all clean. resolveStringCount uses MAX_RENDER_STRINGS
and NSTR — copied as compile-time constants (both = 6) matching IIFE values.
_ssActive/_ssIsCanvasFocused read window.feedBackSplitscreen live per call.

Survey discrepancy declared: the 12 functions span two non-contiguous regions
in current screen.js (lines 741–882 and 1490–1503) rather than the plan's
original 1703–2193 range; function list from the plan is exact.

New test file highway_3d_utils.test.js: 19 class-killer tests.
Mutation-verified (3-char shorthand removal → RED, original → GREEN).
Panel-controls sandbox: 16 stubs added for the new imports.

Suite: node --test tests/js/highway_3d*.test.js plugins/highway_3d/tests/*.test.js
188→207/207 pass.

Plugin: 3.38.0 → 3.39.0

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

plugins/highway_3d/plugin.json

@@ -1,7 +1,7 @@
 {
     "id": "highway_3d",
     "name": "3D Highway",
-    "version": "3.38.0",
+    "version": "3.39.0",
     "type": "visualization",
     "scriptType": "module",
     "bundled": true,