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/queue/hooks/useQueueAutoScroll.ts

@@ -1,6 +1,6 @@
 import React, { useLayoutEffect } from 'react';
-import { registerQueueListScrollTopReader, consumePendingQueueListScrollTop } from '@/store/queueUndo';
-import type { QueueItemRef, Track } from '@/store/playerStoreTypes';
+import { registerQueueListScrollTopReader, consumePendingQueueListScrollTop } from '@/features/playback/store/queueUndo';
+import type { QueueItemRef, Track } from '@/features/playback/store/playerStoreTypes';
 
 interface Args {
   queue: QueueItemRef[];

src/features/queue/hooks/useQueuePanelDrag.ts

@@ -1,9 +1,9 @@
 import React, { useEffect, useRef, useState } from 'react';
 import { resolveAlbum, resolveMediaServerId } from '@/features/offline';
-import { songToTrack } from '@/utils/playback/songToTrack';
+import { songToTrack } from '@/features/playback/utils/playback/songToTrack';
 import { useDragDrop, registerQueueDragHitTest } from '@/lib/dnd/DragDropContext';
-import { usePlayerStore } from '@/store/playerStore';
-import type { Track } from '@/store/playerStoreTypes';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
+import type { Track } from '@/features/playback/store/playerStoreTypes';
 
 /** Drag types that may be dropped into the queue panel. */
 const QUEUE_DROP_TYPES = new Set(['song', 'album', 'queue_reorder']);

src/features/queue/hooks/useQueueTrackEnrichment.ts

@@ -10,7 +10,7 @@ import {
   type ParsedTrackEnrichment,
 } from '@/utils/library/trackEnrichment';
 import { libraryIsReady } from '@/utils/library/libraryReady';
-import { normalizeAnalysisTrackId } from '@/utils/playback/queueIdentity';
+import { normalizeAnalysisTrackId } from '@/features/playback/utils/playback/queueIdentity';
 
 const EMPTY: ParsedTrackEnrichment = {
   serverBpm: null,

src/features/queue/hooks/useQueueTracks.test.ts

@@ -1,7 +1,7 @@
 import { describe, it, expect, beforeEach } from 'vitest';
 import { renderHook } from '@testing-library/react';
-import { usePlayerStore } from '@/store/playerStore';
-import type { Track } from '@/store/playerStoreTypes';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
+import type { Track } from '@/features/playback/store/playerStoreTypes';
 import { getCachedTrack, _resetQueueResolverForTest } from '@/utils/library/queueTrackResolver';
 import { seedQueue } from '@/test/helpers/factories';
 import { useQueueTrackAt, useCurrentTrack, useQueueItems } from '@/features/queue/hooks/useQueueTracks';

src/features/queue/hooks/useQueueTracks.ts

@@ -1,6 +1,6 @@
 import { useMemo, useSyncExternalStore } from 'react';
-import { usePlayerStore } from '@/store/playerStore';
-import type { QueueItemRef, Track } from '@/store/playerStoreTypes';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
+import type { QueueItemRef, Track } from '@/features/playback/store/playerStoreTypes';
 import { resolveQueueTrack } from '@/utils/library/queueTrackView';
 import {
   getQueueResolverVersion,