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/keys_highway_3d/screen.js

@@ -1604,15 +1604,24 @@
      *  Camera Director bridge resolver (pure — exported via createFactory.__test)
      * ====================================================================== */
 
-    // Resolve the active splitscreen API, defensive on the global-name rename in
-    // flight (feedBackSplitscreen is canonical; slopsmithSplitscreen is the legacy
-    // alias). Returns null when splitscreen isn't present.
+    /**
+     * The active splitscreen API, defensive on the global-name rename in flight
+     * (feedBackSplitscreen is canonical; slopsmithSplitscreen is the legacy alias).
+     * @returns {object|null} the splitscreen API, or null when not present
+     */
     function _ssApi() { return window.feedBackSplitscreen || window.slopsmithSplitscreen || null; }
 
-    // Given the splitscreen API, THIS window's per-panel camera map, and the
-    // global camera, return this panel's camera under splitscreen, else the
-    // global, else null (Camera Director absent → 100% stock framing). Throw-safe
-    // on panelIndexFor so a misbehaving splitscreen build can't break framing.
+    /**
+     * Resolve the Camera Director camera for a canvas: this panel's camera under
+     * splitscreen, else the global, else null (Camera Director absent → 100% stock
+     * framing). Throw-safe on panelIndexFor so a misbehaving splitscreen build
+     * can't break framing.
+     * @param {HTMLCanvasElement} canvas this renderer's highway canvas
+     * @param {object|null} ss the splitscreen API (see _ssApi)
+     * @param {object|null} panelsMap window.__h3dCamCtlPanels (per-panel cameras by index)
+     * @param {object|null} globalCam window.__h3dCamCtl (single global camera)
+     * @returns {object|null} the resolved free-camera bridge, or null
+     */
     function _resolveFreeCam(canvas, ss, panelsMap, globalCam) {
         if (panelsMap && ss && typeof ss.panelIndexFor === 'function') {
             try {

plugins/keys_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);