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

tests/js/highway_3d_panel_controls.test.js

@@ -75,6 +75,23 @@ function loadHighway3dStatics() {
         // h3d-carve-2: T and loadThree moved to src/three-loader.js.
         T: null,
         loadThree: () => Promise.resolve(),
+        // h3d-carve-3: color/tuning/splitscreen utils moved to src/utils.js.
+        _h3dHexToInt: () => null,
+        _clampByteI: n => n,
+        _darkenInt: (hex) => hex,
+        _lightenInt: (hex) => hex,
+        resolveStringCount: () => 6,
+        _NOTE_NAMES_SHARP: ['C', 'C#', 'D', 'D#', 'E', 'F', 'F#', 'G', 'G#', 'A', 'A#', 'B'],
+        _BASE_OPEN_MIDI_BASS4: Object.freeze([28, 33, 38, 43]),
+        _BASE_OPEN_MIDI_BASS5: Object.freeze([23, 28, 33, 38, 43]),
+        _BASE_OPEN_MIDI_GUITAR6: Object.freeze([40, 45, 50, 55, 59, 64]),
+        _BASE_OPEN_MIDI_GUITAR7: Object.freeze([35, 40, 45, 50, 55, 59, 64]),
+        _BASE_OPEN_MIDI_GUITAR8: Object.freeze([28, 35, 40, 45, 50, 55, 59, 64]),
+        _baseOpenStringMidis: () => [40, 45, 50, 55, 59, 64],
+        _midiToPitchLabel: () => 'A4',
+        _openStringPitchLabelsForTuning: () => [],
+        _ssActive: () => false,
+        _ssIsCanvasFocused: () => true,
     };
     vm.createContext(sandbox);
     vm.runInContext(instrumented, sandbox, { filename: SCREEN_JS });