refactor(h3d-carve-1): extract pure geometry helpers to src/geometry.js

Moves 7 exports from screen.js to a new ES module src/geometry.js:
  geoFretX (was fretX, now 2-arg), dZ, slideTrailEnd, camBaseDistU,
  camLowFretPullbackU, computeBPM, _makeGaussTex.

Internal helpers _fretXLog / _fretXUniStep / _fretXUni are module-private
inside geometry.js. Module-level constants (SCALE, K, FRET_SCALE, NFRETS,
FRET_SPACING_*, TS) are duplicated at geometry.js module scope — values
are compile-time and never vary at runtime.

screen.js keeps a 1-arg delegator (1 beyond-subst):
  const fretX = f => geoFretX(f, _h3dFretUniform);
No call site changes; fretMid / fretColumnWorldW / slideOffsetWorldX /
_recomputeFretSpacingDerived stay in screen.js and use the delegator.

Tests:
- highway_3d_geometry.test.js: class-killer via import() with 9
  known-answer assertions (geoFretX uniform/log invariants, dZ linearity,
  slideTrailEnd, computeBPM BPM estimate).
- highway_3d_fret_spacing: test updated to match delegator pattern + adds
  GEOMETRY_JS read to verify geoFretX export.
- highway_3d_sustain_bloom: retargeted _makeGaussTex check to GEOMETRY_JS
  (definition moved); call-site check stays on SCREEN_JS.
- highway_3d_panel_controls: vm loader strips ES import line and injects
  geometry stubs; all factory-statics tests unaffected.

Suite: 169/169 pass
  node --test tests/js/highway_3d*.test.js plugins/highway_3d/tests/*.test.js
Version bump: 3.35.0 -> 3.36.0

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": "3.35.0",
+    "version": "3.36.0",
     "type": "visualization",
     "scriptType": "module",
     "bundled": true,