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.test.ts

@@ -0,0 +1,32 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+const { touchPlayedMock } = vi.hoisted(() => ({
+  touchPlayedMock: vi.fn(),
+}));
+
+vi.mock('./hotCacheStore', () => ({
+  useHotCacheStore: { getState: () => ({ touchPlayed: touchPlayedMock }) },
+}));
+
+import { touchHotCacheOnPlayback } from './hotCacheTouch';
+
+beforeEach(() => {
+  touchPlayedMock.mockClear();
+});
+
+describe('touchHotCacheOnPlayback', () => {
+  it('forwards a populated id pair to the hot-cache store', () => {
+    touchHotCacheOnPlayback('t1', 'srv');
+    expect(touchPlayedMock).toHaveBeenCalledWith('t1', 'srv');
+  });
+
+  it('skips when the trackId is empty', () => {
+    touchHotCacheOnPlayback('', 'srv');
+    expect(touchPlayedMock).not.toHaveBeenCalled();
+  });
+
+  it('skips when the serverId is empty', () => {
+    touchHotCacheOnPlayback('t1', '');
+    expect(touchPlayedMock).not.toHaveBeenCalled();
+  });
+});