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/api/coverCache.test.ts

@@ -1,6 +1,6 @@
 import { beforeEach, describe, expect, it } from 'vitest';
 import { useAuthStore } from '../store/authStore';
-import { usePlayerStore } from '../store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import { makeTrack } from '../test/helpers/factories';
 import { resetAllStores } from '../test/helpers/storeReset';
 import { invokeMock, onInvoke } from '../test/mocks/tauri';

src/api/coverCache.ts

@@ -4,7 +4,7 @@ import { useThemeStore } from '../store/themeStore';
 import { coverIndexKeyFromRef, coverStorageKeyFromRef } from '../cover/storageKeys';
 import { connectBaseUrlForServer } from '../utils/server/serverEndpoint';
 import { serverIndexKeyForProfile } from '../utils/server/serverIndexKey';
-import { getPlaybackServerId } from '../utils/playback/playbackServer';
+import { getPlaybackServerId } from '@/features/playback/utils/playback/playbackServer';
 import { restBaseFromUrl } from '@/lib/api/subsonicClient';
 import type { CoverArtRef, CoverArtTier } from '../cover/types';