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/hotCacheTouch.ts

@@ -0,0 +1,14 @@
+import { useHotCacheStore } from './hotCacheStore';
+
+/**
+ * Mark a track as recently played for the hot-cache LRU. Called from every
+ * `audio_play` entry point — cold start, gapless switch, queue rewrite,
+ * radio next — so the hot cache promotes frequently-played tracks even when
+ * playback bounced through different code paths. The empty-id guards keep
+ * dev-time crashes (e.g. unauthenticated state, server still resolving)
+ * from surfacing as cache writes against a meaningless key.
+ */
+export function touchHotCacheOnPlayback(trackId: string, serverId: string): void {
+  if (!trackId || !serverId) return;
+  useHotCacheStore.getState().touchPlayed(trackId, serverId);
+}