refactor(playback): move the audio engine into features/playback

Relocate the playback/queue/transport/audio-output engine out of the type-first
store/ + utils/playback/ + utils/audio/ dirs into a cohesive src/features/playback/,
structure-preserving:
  store/<x>                    -> features/playback/store/<x>
  store/audioListenerSetup/<x> -> features/playback/store/audioListenerSetup/<x>
  utils/playback/<x>           -> features/playback/utils/playback/<x>
  utils/audio/<x>              -> features/playback/utils/audio/<x>

184 files moved (107 source + 77 tests), 365 consumers rewritten. Pure move — no
behavior change, no state-split (the playerStore state-split stays a separate M5
question). Enabled by this session's decouple seams (artist/offline/orbit/auth →
core registries), so the engine carries no inbound core->feature inversion: store/
now holds only the 50 cross-cutting global stores (auth family, the seams, library
index, UI/settings stores).

KEPT OUT of the move (would re-create global->engine edges): the 3 pure config
helpers utils/audio/{loudnessPreAnalysisSlider,hiResCrossfadeResample} +
utils/playback/autodjOverlapCap (authStore + settings UI read them — they stay in
utils/). Ambiguous view-state stores (eqStore, queueToolbarStore,
playerBarLayoutStore) stay global (no engine imports).

Consumers use DEEP paths (@/features/playback/...), no barrel — matches the lib/
approach and avoids barrel-mock-collapse across the 140 usePlayerStore consumers.
Two tolerated type-only core->feature edges remain (localPlaybackStore->QueueItemRef,
localPlaybackMigration->HotCacheEntry, both erased).

tsc 0, lint 0, full suite 319/2353 green, iron-rule clean (no runtime store->feature
import). Behavior-touching only via the prerequisite bridge seam (already QA-flagged);
the move itself is pure.

src/features/playback/store/timelineSessionHistory.test.ts

@@ -0,0 +1,65 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import {
+  _resetTimelineSessionHistoryForTest,
+  appendTimelineSessionPlay,
+  applyTimelineBootstrap,
+  clearTimelineSessionHistory,
+  getTimelineSessionHistorySnapshot,
+  isTimelineBootstrapAttempted,
+  isTimelineHistoryClearedThisSession,
+  markTimelineBootstrapAttempted,
+  TIMELINE_APPEND_DEDUPE_MS,
+  TIMELINE_MERGE_DEDUPE_MS,
+} from '@/features/playback/store/timelineSessionHistory';
+
+describe('timelineSessionHistory', () => {
+  beforeEach(() => {
+    _resetTimelineSessionHistoryForTest();
+  });
+
+  it('appends and dedupes within append window', () => {
+    const t0 = 1_000_000;
+    vi.spyOn(Date, 'now').mockReturnValue(t0);
+    appendTimelineSessionPlay({ serverId: 's1', trackId: 'a', playedAtMs: t0 });
+    appendTimelineSessionPlay({ serverId: 's1', trackId: 'a', playedAtMs: t0 + 500 });
+    expect(getTimelineSessionHistorySnapshot()).toHaveLength(1);
+
+    appendTimelineSessionPlay({ serverId: 's1', trackId: 'a', playedAtMs: t0 + TIMELINE_APPEND_DEDUPE_MS + 1 });
+    expect(getTimelineSessionHistorySnapshot()).toHaveLength(2);
+    vi.restoreAllMocks();
+  });
+
+  it('seeds bootstrap when buffer empty', () => {
+    applyTimelineBootstrap([
+      { serverId: 's1', trackId: 'a', playedAtMs: 100 },
+      { serverId: 's1', trackId: 'b', playedAtMs: 200 },
+    ]);
+    expect(getTimelineSessionHistorySnapshot()).toEqual([
+      { serverId: 's1', trackId: 'a', playedAtMs: 100 },
+      { serverId: 's1', trackId: 'b', playedAtMs: 200 },
+    ]);
+  });
+
+  it('merge prepends older bootstrap rows before live appends', () => {
+    appendTimelineSessionPlay({ serverId: 's1', trackId: 'live', playedAtMs: 5_000 });
+    applyTimelineBootstrap([
+      { serverId: 's1', trackId: 'old', playedAtMs: 1_000 },
+      { serverId: 's1', trackId: 'live', playedAtMs: 5_000 - TIMELINE_MERGE_DEDUPE_MS },
+      { serverId: 's1', trackId: 'mid', playedAtMs: 3_000 },
+    ]);
+    expect(getTimelineSessionHistorySnapshot().map(r => r.trackId)).toEqual(['old', 'mid', 'live']);
+  });
+
+  it('clear blocks bootstrap merge', () => {
+    clearTimelineSessionHistory();
+    expect(isTimelineHistoryClearedThisSession()).toBe(true);
+    applyTimelineBootstrap([{ serverId: 's1', trackId: 'a', playedAtMs: 1 }]);
+    expect(getTimelineSessionHistorySnapshot()).toHaveLength(0);
+  });
+
+  it('bootstrap attempted only once', () => {
+    expect(markTimelineBootstrapAttempted()).toBe(true);
+    expect(markTimelineBootstrapAttempted()).toBe(false);
+    expect(isTimelineBootstrapAttempted()).toBe(true);
+  });
+});