test(playerStore): characterize pure helpers + queue mutations (Phase F1 / PR 2a) (#540)

trackShape.test.ts: songToTrack mapping (required, optional, replayGain
flattening, missing-block fallback, no invented flags); resolveReplayGainDb
precedence (disabled, track, album, auto with prev/next neighbour match,
missing albumId fallback, both gains missing); shuffleArray (non-mutating,
preserves multiset, copies, empty/single, deterministic under mocked RNG).

queue.test.ts: enqueue / enqueueAt (auto-added separator placement,
queueIndex shift, index clamping), playNext (playNextAdded tag, position,
empty input), clearQueue (state reset, audio_stop call), reorderQueue
(splice, currentTrack-id-following queueIndex), removeTrack (clamp on
shrink, keep when after cursor), undo / redo (empty stacks return false,
rollback, replay, new edit drops pending redo).

Adds a 5-line test-only export _resetQueueUndoStacksForTest in playerStore.ts
because the undo/redo arrays live at module scope outside the Zustand state
graph; storeReset.ts now calls it so resetPlayerStore is fully complete.

playerStore.ts coverage 4.76% -> 18.1% lines. PR 2b + 2c push further toward
the F1 floor of 50%.

src/test/helpers/storeReset.ts

@@ -20,7 +20,7 @@
  *   // or for cross-store tests:
  *   beforeEach(resetAllStores);
  */
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore, _resetQueueUndoStacksForTest } from '@/store/playerStore';
 import { useAuthStore } from '@/store/authStore';
 import { usePreviewStore } from '@/store/previewStore';
 import { useOrbitStore } from '@/store/orbitStore';
@@ -32,6 +32,8 @@ const INITIAL_ORBIT_STATE = useOrbitStore.getState();
 
 export function resetPlayerStore(): void {
   usePlayerStore.setState(INITIAL_PLAYER_STATE, true);
+  // Module-scoped queue undo/redo stacks live outside the Zustand state.
+  _resetQueueUndoStacksForTest();
 }
 
 export function resetAuthStore(): void {