fix(h3d): cut-16 follow-up — tests, plugin.json bump, visibility/string-colors retargets

- Add tests/js/highway_3d_scene_init.test.js (18 tests): wiring guard,
  export surface, class-killer, DI anti-vacuity, import correctness,
  function presence, kill tests, plugin.json version gate.
- Bump plugins/highway_3d/plugin.json 3.51.0 → 3.52.0.
- Retarget highway_string_colors.test.js tests 3–4 to scene-init.js
  (functions moved in cut-16).
- Update highway_visibility.test.js test 22 regexes to accept DI-rewritten
  forms: getHighwayCanvas() / getWrap().style.display / getHighwayCanvas().offsetParent.

Suite: 1426/1427 pass (1 pre-existing audio test; all cut-16 regressions cleared).

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": "3.51.0",
+  "version": "3.52.0",
   "type": "visualization",
   "scriptType": "module",
   "bundled": true,