h3d-carve-13: extract S-section lookahead helpers into src/camera.js

Partial cut (Option A) per contract ~/Feedback-harness/hive/plans/h3d-cut13-contract.md.

What moves:
- lookaheadEndTime (factory-private) — 1 beyond-subst: _measureStarts → getMeasureStarts()
- lookaheadBootstrapTime (exported)
- lookaheadComputeFretBounds (exported) — uses lowerBoundT from geometry.js (new import)
- lookaheadTargetWorldX (exported)

DI delta: +9 params to createCamera()
  Constants (+4): NFRETS=24, CAM_LOOKAHEAD_MEASURES=9, CAM_LOOKAHEAD_SEC=3.0, CAM_FRET_EDGE_BLEND=0.1
  Getter  (+1): getMeasureStarts: () => _measureStarts
  Fn refs (+4): validString, getChartAnchorAt, xFretMid, xFret
Total createCamera() DI: 57 (was 48)

Deferred to cut 15 (U-section entanglement):
  lookaheadSmoothCamStep, _applyNoteCamTargets — write S-section state vars
  Region B (per-frame cam target block, ~90 lines) — reads 8 update() locals
  Region C (song-change + bootstrap block, ~174 lines) — U-section init code

Call sites unchanged (destructured names unchanged):
  lookaheadComputeFretBounds: lines 7474, 7941
  lookaheadBootstrapTime: line 7939
  lookaheadTargetWorldX: lines 7943, 10062

Tests: 17 new (highway_3d_camera_lookahead.test.js), 2 retargeted to cameraSrc.
Suite: 1353/1355 pass (2 pre-existing failures in unrelated test files).

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

tests/js/highway_3d_camera_framing.test.js

@@ -98,27 +98,29 @@ test('measure-start cache only keeps beats with measure >= 0', () => {
 });
 
 test('lookaheadEndTime targets the measure CAM_LOOKAHEAD_MEASURES ahead', () => {
+    // h3d-carve-13: lookaheadEndTime moved to src/camera.js — retarget to cameraSrc.
     assert.match(
-        src,
+        cameraSrc,
         /function\s+lookaheadEndTime\s*\(\s*now\s*\)/,
         'lookaheadEndTime(now) helper must exist',
     );
     assert.match(
-        src,
+        cameraSrc,
         /const\s+targetIdx\s*=\s*curIdx\s*\+\s*CAM_LOOKAHEAD_MEASURES/,
         'target measure index = current measure + CAM_LOOKAHEAD_MEASURES',
     );
     // No beats → seconds fallback.
     assert.match(
-        src,
+        cameraSrc,
         /if\s*\(\s*!ms\s*\|\|\s*ms\.length\s*===\s*0\s*\)\s*return\s+now\s*\+\s*CAM_LOOKAHEAD_SEC/,
         'lookaheadEndTime must fall back to seconds when there are no measures',
     );
 });
 
 test('fret-bounds scan drives its window off lookaheadEndTime, not fixed seconds', () => {
+    // h3d-carve-13: lookaheadComputeFretBounds moved to src/camera.js — retarget to cameraSrc.
     assert.match(
-        src,
+        cameraSrc,
         /function\s+lookaheadComputeFretBounds[\s\S]*?const\s+tEnd\s*=\s*lookaheadEndTime\(\s*now\s*\)/,
         'lookaheadComputeFretBounds must derive tEnd from lookaheadEndTime(now)',
     );