refactor(player): E.11 — extract two playback-coordination helpers (#574)

Two small file-private helpers move into dedicated modules under
src/store/:

- `waveformRefreshGen.ts` — the per-track generation counter that
  guards against applying a stale waveform read after the cache was
  invalidated. Exposes `bumpWaveformRefreshGen` (existing) +
  `getWaveformRefreshGen` (new accessor that replaces the two direct
  reads at `refreshWaveformForTrack`).
- `hotCacheTouch.ts` — `touchHotCacheOnPlayback` with its empty-id
  guards, called from every `audio_play` entry point.

Both file-private; no caller-side changes outside playerStore's own
imports. 8 focused tests pin the generation-increment + isolation
contract and the touch helper's empty-id guards.

playerStore 3302 → 3291 LOC.

src/store/waveformRefreshGen.ts

@@ -0,0 +1,32 @@
+/**
+ * Last-write-wins generation counter per track. Avoids applying a stale
+ * empty waveform read when `analysis:waveform-updated` bumps the gen after
+ * SQLite commit while an older `analysis_get_waveform_for_track` is still
+ * in flight. Gen is bumped only on explicit invalidation (waveform-updated,
+ * analysis storage), not on every `refreshWaveformForTrack` call —
+ * otherwise bursts (Lucky Mix, queue) cancel each other.
+ *
+ * Typical usage:
+ *
+ *   const gen = getWaveformRefreshGen(trackId);
+ *   const row = await invoke('analysis_get_waveform_for_track', { trackId });
+ *   if (getWaveformRefreshGen(trackId) !== gen) return; // stale result
+ */
+
+const waveformRefreshGenByTrackId: Record<string, number> = {};
+
+export function bumpWaveformRefreshGen(trackId: string): void {
+  if (!trackId) return;
+  waveformRefreshGenByTrackId[trackId] = (waveformRefreshGenByTrackId[trackId] ?? 0) + 1;
+}
+
+export function getWaveformRefreshGen(trackId: string): number {
+  return waveformRefreshGenByTrackId[trackId] ?? 0;
+}
+
+/** Test-only: wipe the per-track generations so each spec starts fresh. */
+export function _resetWaveformRefreshGenForTest(): void {
+  for (const k of Object.keys(waveformRefreshGenByTrackId)) {
+    delete waveformRefreshGenByTrackId[k];
+  }
+}