refactor(player): E.18 — extract three transport-coordination modules (#581)

Cluster of three small thematically-related cuts in one PR (per the new
'cluster, don't single-shot' convention):

  - `src/store/seekTargetState.ts` — the seek-target guard (`seekTarget`,
    `seekTargetSetAt`, `SEEK_TARGET_GUARD_TIMEOUT_MS`, set/clear/get
    accessors) that suppresses stale Rust progress ticks until the
    engine catches up to the requested position
  - `src/store/togglePlayLock.ts` — the 300 ms double-click cooldown
    behind a `tryAcquireTogglePlayLock()` helper that auto-releases on
    a timer (collapses the three-line inline pattern in `togglePlay`)
  - `src/store/loudnessReseed.ts` — the full `reseedLoudnessForTrackId`
    pipeline (gen-bump → cache + backfill wipe → state reset → server
    row delete → forced seed enqueue), pulled out of playerStore as a
    single async helper

All three were file-private; no caller-side changes outside
playerStore's own imports. The progress handler's seek-guard branch is
now ~3 lines shorter and reads through accessors. `togglePlay` collapses
to one guard check.

24 tests across the three modules pin the API contracts.

playerStore 3189 → 3139 LOC.

src/store/togglePlayLock.test.ts

@@ -0,0 +1,47 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import {
+  _resetTogglePlayLockForTest,
+  tryAcquireTogglePlayLock,
+} from './togglePlayLock';
+
+beforeEach(() => {
+  vi.useFakeTimers();
+});
+
+afterEach(() => {
+  _resetTogglePlayLockForTest();
+  vi.useRealTimers();
+});
+
+describe('tryAcquireTogglePlayLock', () => {
+  it('returns true on first acquire', () => {
+    expect(tryAcquireTogglePlayLock()).toBe(true);
+  });
+
+  it('returns false when already held', () => {
+    tryAcquireTogglePlayLock();
+    expect(tryAcquireTogglePlayLock()).toBe(false);
+  });
+
+  it('auto-releases after the default 300 ms window', () => {
+    tryAcquireTogglePlayLock();
+    vi.advanceTimersByTime(299);
+    expect(tryAcquireTogglePlayLock()).toBe(false);
+    vi.advanceTimersByTime(1);
+    expect(tryAcquireTogglePlayLock()).toBe(true);
+  });
+
+  it('honours a custom lock duration', () => {
+    tryAcquireTogglePlayLock(1000);
+    vi.advanceTimersByTime(500);
+    expect(tryAcquireTogglePlayLock()).toBe(false);
+    vi.advanceTimersByTime(500);
+    expect(tryAcquireTogglePlayLock()).toBe(true);
+  });
+
+  it('_resetTogglePlayLockForTest force-releases', () => {
+    tryAcquireTogglePlayLock();
+    _resetTogglePlayLockForTest();
+    expect(tryAcquireTogglePlayLock()).toBe(true);
+  });
+});