refactor(h3d-carve-4): extract Butterchurn panel to src/bc-panel.js

Move all _bc* constants, mutable state, and functions (~670 lines) from
screen.js B-section (lines 27-695) to src/bc-panel.js.

Public API: _bcCreateController, _bcIsDesktop (2 named exports).
window.h3dBcApplySettings assigned at bc-panel.js module scope (1
beyond-subst; body verbatim — scope moves from IIFE to ES module top-level).
THREE_URL / THREE_CDN dead-code tombstoned (unused since three-loader.js).
Vendor files untouched (R2); asset URL constants verbatim (R3).

Delta note: original survey estimated ~1,333 lines for B-section. Actual is
~670 because (a) prior cuts 1b/2/3 moved material that was interleaved in the
B-section range, and (b) the original survey section boundaries were wrong —
the B-section ends where the H-section factory begins, not at line 695 of the
pre-cut file.

Completeness grep (post-cut):
  grep -nP '^\s*(function|const|let|var)\s+_bc' screen.js
  → 2 hits: _bcActive and _bcSyncMode at line 8081-8082 (factory-scope
    H/P-section helpers that orchestrate the imported controller; not B-section
    symbols — confirmed F-section in the survey table).
  B-section _bc* definitions in IIFE: 0.

Suite: node --test tests/js/highway_3d*.test.js plugins/highway_3d/tests/*.test.js
  tests 221   pass 221   fail 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": "3.39.0",
+    "version": "3.40.0",
     "type": "visualization",
     "scriptType": "module",
     "bundled": true,