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/screen.js

@@ -6,6 +6,8 @@
 // main player and per-panel in splitscreen without any architectural
 // changes.
 
+import { geoFretX, dZ, slideTrailEnd, camBaseDistU, camLowFretPullbackU, computeBPM, _makeGaussTex } from './src/geometry.js'; // h3d-carve-1
+
 (function () {
     'use strict';
 
@@ -1510,22 +1512,11 @@
      *  Pure helpers
      * ====================================================================== */
 
-    // Logarithmic spacing — mirrors real guitar fret geometry (12th root of 2).
-    const _fretXLog = f => {
-        if (f <= 0) return 0;
-        const raw = FRET_SCALE - FRET_SCALE / Math.pow(2, f / 12);
-        if (f <= FRET_SPACING_ANCHOR_F) return raw;
-        const rawAnchor = FRET_SCALE - FRET_SCALE / Math.pow(2, FRET_SPACING_ANCHOR_F / 12);
-        return rawAnchor + (raw - rawAnchor) * FRET_SPACING_STRETCH_ABOVE12;
-    };
-    // Uniform spacing — same column width per fret (chart-format style).
-    // Total board width equals the logarithmic NFRETS position for consistency.
-    const _fretXUniStep = _fretXLog(NFRETS) / NFRETS;
-    const _fretXUni = f => f <= 0 ? 0 : f * _fretXUniStep;
+    // _fretXLog, _fretXUniStep, _fretXUni — moved to src/geometry.js (h3d-carve-1).
 
     let _h3dFretUniform = true;
     try { _h3dFretUniform = localStorage.getItem('highway_3d.fretSpacing') !== 'logarithmic'; } catch (_) {}
-    const fretX = f => _h3dFretUniform ? _fretXUni(f) : _fretXLog(f);
+    const fretX = f => geoFretX(f, _h3dFretUniform); // h3d-carve-1: delegator (1 beyond-subst)
 
     window.h3dSetFretSpacing = mode => {
         // Validate against the two supported modes before persisting so an
@@ -1568,24 +1559,9 @@
         const m = w / _fretLabelScaleRefW;
         return Math.max(0.32, Math.min(1.45, m));
     }
-    const dZ = dt => -dt * TS;
+    // dZ — moved to src/geometry.js (h3d-carve-1).
 
-    /**
-     * Pitched slide uses `sl`, unpitched uses `slu` (slide-to vs unpitched slide fields).
-     * Prefer `sl` when both are present — matches RS wire.
-     * @returns {{ endFret: number, unpitched: boolean } | null}
-     */
-    function slideTrailEnd(n) {
-        const sl = n.sl;
-        const slu = n.slu;
-        if (Number.isFinite(sl) && sl >= 0) {
-            return { endFret: sl | 0, unpitched: false };
-        }
-        if (Number.isFinite(slu) && slu >= 0) {
-            return { endFret: slu | 0, unpitched: true };
-        }
-        return null;
-    }
+    // slideTrailEnd — moved to src/geometry.js (h3d-carve-1).
 
     /**
      * Lateral slide offset along the fretboard during sustain — easing
@@ -1605,15 +1581,7 @@
         return (endX - startX) * w;
     }
 
-    // Camera tgtDist building blocks. Both the dynamic (camera-follow)
-    // and locked (frets 1-12) branches compose tgtDist from these, so
-    // any future tuning of the base zoom curve or low-fret pullback
-    // lands in both branches without drift.
-    //   span    — camDistMax - camDistMin in fret-span units
-    //   minFret — lowest fretted note in the camera window (or 1 for
-    //             the locked branch, which assumes nut chords)
-    const camBaseDistU = span => 65 + Math.max(span, 4) * 3;
-    const camLowFretPullbackU = minFret => Math.max(0, 5 - minFret) * 4;
+    // camBaseDistU, camLowFretPullbackU — moved to src/geometry.js (h3d-carve-1).
 
     // World-units-per-fret near mid-neck. Used by the camera-X hysteresis
     // gate (issue #34) to convert a fret-equivalent dead zone into world
@@ -1632,52 +1600,7 @@
         FRET_WIDTH_MID = fretX(7) - fretX(6);
     }
 
-    function computeBPM(beats, t) {
-        if (!beats || beats.length < 2) return 120;
-        let lo = 0, hi = beats.length;
-        while (lo < hi) {
-            const mid = (lo + hi) >> 1;
-            if (beats[mid].time < t) lo = mid + 1; else hi = mid;
-        }
-        let closest = lo;
-        if (lo === beats.length) closest = beats.length - 1;
-        else if (lo > 0 && Math.abs(beats[lo - 1].time - t) < Math.abs(beats[lo].time - t)) closest = lo - 1;
-        const start = Math.max(0, closest - 2);
-        const end = Math.min(beats.length - 1, closest + 2);
-        let sum = 0, count = 0;
-        for (let i = start; i < end; i++) {
-            const dt = beats[i + 1].time - beats[i].time;
-            if (dt > 0) { sum += dt; count++; }
-        }
-        return count > 0 && sum > 0 ? 60 / (sum / count) : 120;
-    }
-
-    // Build a horizontal gaussian DataTexture for the sustain-rail bloom effect.
-    // Returns a W×1 RGBA texture where alpha follows exp(-0.5*(u−0.5)²/σ²),
-    // peaking at 1.0 in the centre. With the default σ=0.28 the edges retain
-    // ~0.20 alpha (not fully transparent) — a deliberately soft, wide falloff
-    // so the additive bloom fades gradually rather than cutting off sharply.
-    // Power-of-two width keeps WebGL mipmapping happy.
-    function _makeGaussTex(ThreeLib, w = 128, sigma = 0.28) {
-        const data = new Uint8Array(w * 4);
-        for (let i = 0; i < w; i++) {
-            const u = i / (w - 1);
-            const d = (u - 0.5) / sigma;
-            const v = Math.exp(-0.5 * d * d);
-            const a = Math.round(v * 255);
-            data[i * 4]     = 255;
-            data[i * 4 + 1] = 255;
-            data[i * 4 + 2] = 255;
-            data[i * 4 + 3] = a;
-        }
-        const tex = new ThreeLib.DataTexture(data, w, 1, ThreeLib.RGBAFormat);
-        // LinearFilter on both axes so the bloom plane interpolates smoothly
-        // when scaled — the default NearestFilter causes visible banding.
-        tex.magFilter = ThreeLib.LinearFilter;
-        tex.minFilter = ThreeLib.LinearFilter;
-        tex.needsUpdate = true;
-        return tex;
-    }
+    // computeBPM, _makeGaussTex — moved to src/geometry.js (h3d-carve-1).
 
     /* ======================================================================
      *  Three.js module — lazily loaded, memoized