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/store/playerStore.ts

@@ -457,6 +457,12 @@ type QueueUndoSnapshot = {
 const queueUndoStack: QueueUndoSnapshot[] = [];
 const queueRedoStack: QueueUndoSnapshot[] = [];
 
+/** Test-only: clears module-scoped undo/redo stacks so each test starts clean. */
+export function _resetQueueUndoStacksForTest(): void {
+  queueUndoStack.length = 0;
+  queueRedoStack.length = 0;
+}
+
 /** QueuePanel registers a reader so undo snapshots capture list scroll position. */
 let queueListScrollTopReader: (() => number | undefined) | null = null;