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/fullscreenPlayer/components/FsLyricsApple.tsx

@@ -1,11 +1,11 @@
 import React, { memo, useCallback, useEffect, useRef, useState } from 'react';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import { useAuthStore } from '@/store/authStore';
 import { useLyrics, type WordLyricsLine } from '@/hooks/useLyrics';
 import { useWordLyricsSync } from '@/hooks/useWordLyricsSync';
-import { getPlaybackProgressSnapshot, subscribePlaybackProgress } from '@/store/playbackProgress';
+import { getPlaybackProgressSnapshot, subscribePlaybackProgress } from '@/features/playback/store/playbackProgress';
 import type { LrcLine } from '@/api/lrclib';
-import type { Track } from '@/store/playerStoreTypes';
+import type { Track } from '@/features/playback/store/playerStoreTypes';
 import { EaseScroller, targetForFraction } from '@/utils/ui/easeScroll';
 
 // Fullscreen synced lyrics.

src/features/fullscreenPlayer/components/FsPlayBtn.tsx

@@ -1,7 +1,7 @@
 import React, { memo, useRef } from 'react';
 import { Moon, Pause, Play, Sunrise } from 'lucide-react';
 import { useTranslation } from 'react-i18next';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import { usePlaybackDelayPress } from '@/hooks/usePlaybackDelayPress';
 import { usePlaybackScheduleRemaining } from '@/utils/format/playbackScheduleFormat';
 import PlaybackDelayModal from '@/components/PlaybackDelayModal';

src/features/fullscreenPlayer/components/FsQueueModal.tsx

@@ -1,8 +1,8 @@
 import { memo, useMemo, useSyncExternalStore } from 'react';
 import { useTranslation } from 'react-i18next';
 import { X } from 'lucide-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 { resolveQueueTrack } from '@/utils/library/queueTrackView';
 import {
   getQueueResolverVersion,

src/features/fullscreenPlayer/components/FsTimeReadout.tsx

@@ -1,5 +1,5 @@
 import { memo, useEffect, useRef } from 'react';
-import { getPlaybackProgressSnapshot, subscribePlaybackProgress } from '@/store/playbackProgress';
+import { getPlaybackProgressSnapshot, subscribePlaybackProgress } from '@/features/playback/store/playbackProgress';
 import { formatTrackTime } from '@/lib/format/formatDuration';
 
 /**

src/features/fullscreenPlayer/components/FullscreenPlayerStatic.tsx

@@ -4,8 +4,8 @@ import {
   Shuffle, ListMusic, ChevronDown, Star, MicVocal,
 } from 'lucide-react';
 import { useTranslation } from 'react-i18next';
-import { usePlayerStore } from '@/store/playerStore';
-import { queueSongStar, queueSongRating } from '@/store/pendingStarSync';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
+import { queueSongStar, queueSongRating } from '@/features/playback/store/pendingStarSync';
 import { useAlbumCoverRef, useArtistCoverRef } from '@/cover/useLibraryCoverRef';
 import { usePlaybackCoverArt } from '@/hooks/usePlaybackCoverArt';
 import { useCachedUrl } from '@/ui/CachedImage';