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

tests/js/highway_3d_sustain_bloom.test.js

@@ -5,6 +5,10 @@
 // stops using additive blending, or bumps the bloom renderOrder above the
 // core rail (16) would silently regress or invert the effect.
 //
+// Since h3d-carve-1, _makeGaussTex is defined in src/geometry.js and
+// imported into screen.js; the call site (_bloomGaussTex = _makeGaussTex(...))
+// remains in screen.js.
+//
 // Source-level only — same strategy as the other tests/js/ files.
 
 const { test } = require('node:test');
@@ -13,14 +17,16 @@ const fs = require('node:fs');
 const path = require('node:path');
 
 const SCREEN_JS = path.join(__dirname, '..', '..', 'plugins', 'highway_3d', 'screen.js');
+const GEOMETRY_JS = path.join(__dirname, '..', '..', 'plugins', 'highway_3d', 'src', 'geometry.js');
 
 test('a gaussian DataTexture helper (_makeGaussTex) drives the bloom falloff', () => {
-    const src = fs.readFileSync(SCREEN_JS, 'utf8');
+    const geo = fs.readFileSync(GEOMETRY_JS, 'utf8');
     assert.match(
-        src,
-        /function\s+_makeGaussTex\s*\(/,
-        '_makeGaussTex must exist to build the bloom gaussian texture',
+        geo,
+        /export\s+function\s+_makeGaussTex\s*\(/,
+        '_makeGaussTex must be exported from geometry.js to build the bloom gaussian texture',
     );
+    const src = fs.readFileSync(SCREEN_JS, 'utf8');
     assert.match(
         src,
         /_bloomGaussTex\s*=\s*_makeGaussTex\(/,