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_panel_controls.test.js

@@ -28,7 +28,11 @@ function loadHighway3dStatics() {
         1,
         'expected exactly one factory-registration anchor in screen.js',
     );
-    const instrumented = src.replace(
+    // Since h3d-carve-1 screen.js starts with an ES module import statement.
+    // vm.runInContext does not support static import — strip the import line
+    // and provide stub implementations of the geometry exports in the sandbox.
+    const stripped = src.replace(/^import\s+\{[^}]+\}\s+from\s+['"][^'"]+['"];\s*\/\/[^\n]*\n/m, '');
+    const instrumented = stripped.replace(
         ANCHOR,
         `${ANCHOR}\n    window.__h3dTestExports = { BG_DEFAULTS };`,
     );
@@ -50,6 +54,15 @@ function loadHighway3dStatics() {
                 register() {},
             },
         },
+        // Geometry stubs — panel-controls test only reads factory statics;
+        // it never invokes the render path where these are called (h3d-carve-1).
+        geoFretX: (f, _uniform) => f * 0.1,
+        dZ: dt => -dt,
+        slideTrailEnd: () => null,
+        camBaseDistU: span => span,
+        camLowFretPullbackU: () => 0,
+        computeBPM: () => 120,
+        _makeGaussTex: () => ({}),
     };
     vm.createContext(sandbox);
     vm.runInContext(instrumented, sandbox, { filename: SCREEN_JS });