refactor(player): E.23 — extract engine state + seek debounce cluster (#587)

Two thematic cuts in one PR:

  - `src/store/engineState.ts` — `isAudioPaused` (warm-vs-cold-resume
    decision flag) + `playGeneration` (monotonically bumped guard
    counter used by long-running async callbacks to detect concurrent
    playTrack and bail). Get/set accessors + `bumpPlayGeneration()`
    that returns the new value (mirrors the original `++playGeneration`
    pattern).
  - `src/store/seekDebounce.ts` — seek-slider debounce timer behind
    `armSeekDebounce(delayMs, onFire)` / `clearSeekDebounce()` /
    `isSeekDebouncePending()`. Collapses the recurring inline
    `if (seekDebounce) { clearTimeout(...); seekDebounce = null; }`
    triple into single calls.

Sed-driven bulk rewrite for the >60 direct-access sites preserved
semantics verbatim (one JSDoc reference to `isAudioPaused` kept as
comment text). 14 tests across the two modules.

playerStore 2869 → 2867 LOC net (small delta — the imports are bigger
than the savings, but the mutables are no longer reachable from
outside their owning modules).

src/store/engineState.test.ts

@@ -0,0 +1,54 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import {
+  _resetEngineStateForTest,
+  bumpPlayGeneration,
+  getIsAudioPaused,
+  getPlayGeneration,
+  setIsAudioPaused,
+} from './engineState';
+
+afterEach(() => {
+  _resetEngineStateForTest();
+});
+
+describe('isAudioPaused', () => {
+  it('starts false', () => {
+    expect(getIsAudioPaused()).toBe(false);
+  });
+
+  it('round-trips through get/set', () => {
+    setIsAudioPaused(true);
+    expect(getIsAudioPaused()).toBe(true);
+    setIsAudioPaused(false);
+    expect(getIsAudioPaused()).toBe(false);
+  });
+});
+
+describe('playGeneration', () => {
+  it('starts at 0', () => {
+    expect(getPlayGeneration()).toBe(0);
+  });
+
+  it('bumpPlayGeneration increments + returns the new value', () => {
+    expect(bumpPlayGeneration()).toBe(1);
+    expect(bumpPlayGeneration()).toBe(2);
+    expect(getPlayGeneration()).toBe(2);
+  });
+
+  it('captures a snapshot that a later bump invalidates', () => {
+    const snap = bumpPlayGeneration();
+    bumpPlayGeneration();
+    expect(getPlayGeneration()).not.toBe(snap);
+  });
+});
+
+describe('_resetEngineStateForTest', () => {
+  it('resets both fields', () => {
+    setIsAudioPaused(true);
+    bumpPlayGeneration();
+    bumpPlayGeneration();
+    _resetEngineStateForTest();
+    expect(getIsAudioPaused()).toBe(false);
+    expect(getPlayGeneration()).toBe(0);
+  });
+});