fix(highway_3d): _bgPanelKey rejects non-integer panel index; JSDoc bridge fns

- _bgPanelKey() treated any non-null panelIndexFor() return as a valid panel id,
  so a NaN/non-finite index minted a bogus "panelNaN" localStorage key instead
  of falling back to "main". Gate on Number.isInteger(idx) && idx >= 0. (The
  camera path is already NaN-safe — panelsMap[NaN] misses and falls through.)
- Add a NaN/negative-index case to the resolver tests (drum 22, keys 57, pass).
- Convert the camera-bridge helpers' comments to JSDoc (_bgPanelKey, _freeCamFor,
  _resolveFreeCam, _ssApi across the three plugins) to lift docstring coverage on
  the changed surface. Comment/robustness only; no behavior change beyond the
  NaN guard.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Kris Anderson <topkoa@gmail.com>

plugins/drum_highway_3d/tests/camera_bridge.test.js

@@ -56,6 +56,13 @@ test('_resolveFreeCam: throw-safe on panelIndexFor → falls back to global', ()
     assert.equal(__test._resolveFreeCam({}, ss, { 0: {} }, g), g);
 });
 
+test('_resolveFreeCam: NaN/invalid panel index → falls back to global', () => {
+    const { __test } = load();
+    const g = { id: 'global' };
+    assert.equal(__test._resolveFreeCam({}, { panelIndexFor: () => NaN }, { 0: {} }, g), g);
+    assert.equal(__test._resolveFreeCam({}, { panelIndexFor: () => -1 }, { 0: {} }, g), g);
+});
+
 test('_ssApi: null when neither global set; slopsmith alias; feedBack canonical wins', () => {
     const { window, __test } = load();
     assert.equal(__test._ssApi(), null);