review: address PR #694 findings

- isVisible() forces a fresh DOM sample (was serving the rAF loop's
  throttled cache, contradicting its 'live DOM check' docstring).
- v3 chrome: reconcile the edge-driven overControls hover flag against
  matches(':hover') on the throttled ~6 Hz tick — covers a missed
  mouseleave (flag stuck true, transport never hides) and a re-created
  #player-controls node with lost listeners.
- highway_3d pre-warm now also covers teachFg/teachSd label textures
  and the technique sprite factories (mute X, hammer/pull triangles,
  bend chevrons, slide arrows) per active-palette string colour, plus
  a maintenance note tying new label styles to the warm list.
- Document that the visibility throttle's manual invalidations are
  latency-only (periodic resample self-heals within ~10 frames), and
  why highway_3d keeps its local lowerBoundT (downlevel hosts).

External-repo audit (finding 1): staffview, tabview, piano, drums,
keys_highway_3d, drum_highway_3d grepped — no cross-frame bundle
retention or bundle-identity checks found.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

static/v3/player-chrome.js

@@ -291,6 +291,16 @@
             // Re-sync the lyrics icon so programmatic highway.setLyricsVisible()
             // (e.g. from lyrics_karaoke) isn't left stale; cheap + idempotent.
             syncLyricsIcon();
+            // Reconcile the edge-driven hover flag against ground truth at
+            // this throttled cadence (~6 Hz, not per frame). Covers both
+            // failure modes of pure mouseenter/mouseleave tracking: a
+            // missed mouseleave (controls hidden/detached under the
+            // pointer → flag stuck true, transport never auto-hides) and
+            // a re-created #player-controls node whose listeners were
+            // lost (flag stuck false-ish / dead). matches(':hover') on a
+            // detached node is simply false, so this also self-clears.
+            const c = $('player-controls');
+            overControls = !!(c && typeof c.matches === 'function' && c.matches(':hover'));
         }
         tickIdle();
         rafId = requestAnimationFrame(loop);