refactor(h3d-carve-2): extract Three.js loader to src/three-loader.js

Move the D-section (~21 lines) from screen.js factory scope to its own module.

Exports:
- loadThree() — memoised import() with local-vendor→CDN fallback; same body verbatim
- T (live let-binding) — updated to the Three.js namespace on first resolution

screen.js gains: import { loadThree, T } from './src/three-loader.js'
screen.js loses: let T = null; let threeLoadPromise = null; function loadThree()
T and loadThree remain accessible to the IIFE via module-scope closure. The live
let-binding means the IIFE reads the populated T after loadThree() resolves
without any call-site changes.

Panel-controls vm test: strip regex extended to consume all consecutive import
lines; T: null + loadThree stubs added to sandbox context.

Class-killer tests (8 new — highway_3d_three_loader.test.js):
- loadThree exported (mutation: rename/remove → import fails)
- T exported as mutable let (mutation: const → T=mod throws TypeError)
- T=mod in both .then handlers (mutation: remove both → T stays null)
- memoisation guard !threeLoadPromise (mutation: remove → race + duplicate loads)
- CDN fallback .catch chain (mutation: remove → deploy failures unrecoverable)
- threeLoadPromise reset on failure (mutation: remove → no retry possible)
- screen.js imports loadThree+T (wiring confirmed)
- IIFE no longer declares local T or threeLoadPromise (shadow defeated)

All class-killer mutations confirmed distinguishable before commit.

Base run (5e401af): 180/180 pass.
Post-cut run: 188/188 pass.
Command: node --test tests/js/highway_3d*.test.js plugins/highway_3d/tests/*.test.js

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

plugins/highway_3d/screen.js

@@ -7,6 +7,7 @@
 // changes.
 
 import { geoFretX, dZ, slideTrailEnd, camBaseDistU, camLowFretPullbackU, computeBPM, _makeGaussTex, RENDER_ORDER_LAYER_STACK, RENDER_ORDER_LAYER_INDEX, RENDER_ORDER_AT_Z_ZERO, RENDER_ORDER_FAR_CLAMP, renderOrderForLayerAtZ, _noteKey, lowerBoundT, hwyFirstRelevantFrettedTime, geoFretMid } from './src/geometry.js'; // h3d-carve-1b
+import { loadThree, T } from './src/three-loader.js'; // h3d-carve-2
 
 (function () {
     'use strict';
@@ -1479,26 +1480,8 @@ import { geoFretX, dZ, slideTrailEnd, camBaseDistU, camLowFretPullbackU, compute
 
     // computeBPM, _makeGaussTex — moved to src/geometry.js (h3d-carve-1).
 
-    /* ======================================================================
-     *  Three.js module — lazily loaded, memoized
-     * ====================================================================== */
-
-    let T = null;
-    let threeLoadPromise = null;
-    function loadThree() {
-        if (!threeLoadPromise) {
-            threeLoadPromise = import(THREE_URL)
-                .then(mod => { T = mod; return mod; })
-                .catch(() => import(THREE_CDN)
-                    .then(mod => { T = mod; return mod; })
-                    .catch(e => {
-                        console.error('[3D-Hwy] Three.js load failed:', e);
-                        threeLoadPromise = null;
-                        throw e;
-                    }));
-        }
-        return threeLoadPromise;
-    }
+    // T, loadThree — moved to src/three-loader.js (h3d-carve-2).
+    // T is a live-binding export; the IIFE reads the updated value after loadThree() resolves.
 
     /* ======================================================================
      *  Splitscreen helpers