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/search/components/LiveSearch.tsx

@@ -1,6 +1,6 @@
 import { subscribeLibrarySyncIdle, subscribeLibrarySyncProgress } from '@/lib/api/library';
 import type { SearchResults, SubsonicArtist } from '@/lib/api/subsonicTypes';
-import { songToTrack } from '@/utils/playback/songToTrack';
+import { songToTrack } from '@/features/playback/utils/playback/songToTrack';
 import {
   LIVE_SEARCH_DEBOUNCE_NETWORK_MS,
   LIVE_SEARCH_DEBOUNCE_RACE_MS,
@@ -24,7 +24,7 @@ import React, { useState, useEffect, useRef, useCallback } from 'react';
 import { useNavigate, useLocation } from 'react-router-dom';
 import { useNavigateToAlbum } from '@/features/album';
 import { Search, Disc3, Users, Music, TextSearch, Database, Globe } from 'lucide-react';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import { useAuthStore } from '@/store/authStore';
 import { useLibraryIndexStore } from '@/store/libraryIndexStore';
 import { useTranslation } from 'react-i18next';

src/features/search/components/MobileSearchOverlay.tsx

@@ -1,13 +1,13 @@
 import { search } from '@/lib/api/subsonicSearch';
 import type { SearchResults, SubsonicArtist } from '@/lib/api/subsonicTypes';
-import { songToTrack } from '@/utils/playback/songToTrack';
+import { songToTrack } from '@/features/playback/utils/playback/songToTrack';
 import { useLiveSearchScopeStore } from '@/store/liveSearchScopeStore';
 import React, { useState, useEffect, useRef, useCallback, useMemo } from 'react';
 import { createPortal } from 'react-dom';
 import { useLocation, useNavigate } from 'react-router-dom';
 import { navigatePathWithAlbumReturnTo } from '@/utils/navigation/albumDetailNavigation';
 import { X, Search, Disc3, Users, Music, Music2, Clock, ChevronRight } from 'lucide-react';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import { useAuthStore } from '@/store/authStore';
 import { useTranslation } from 'react-i18next';
 import { FETCH_QUEUE_BIAS_SEARCH_ARTIST_OVER_ALBUM } from '@/ui/CachedImage';

src/features/search/components/ShareQueuePreviewModal.tsx

@@ -8,7 +8,7 @@ import { formatTrackTime } from '@/lib/format/formatDuration';
 import type { ShareQueuePreviewState } from '@/features/search/hooks/useShareQueuePreview';
 import { sharePayloadTotal, type QueueableShareSearchPayload } from '@/utils/share/shareSearch';
 import OverlayScrollArea from '@/ui/OverlayScrollArea';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import { COVER_DENSE_SEARCH_CSS_PX } from '@/cover/layoutSizes';
 import { COVER_SCOPE_ACTIVE, type CoverServerScope } from '@/cover/types';
 import { AlbumCoverArtImage } from '@/cover/AlbumCoverArtImage';

src/features/search/components/ShareSearchResults.tsx

@@ -4,7 +4,7 @@ import { useTranslation } from 'react-i18next';
 import type { TFunction } from 'i18next';
 import type { SubsonicArtist } from '@/lib/api/subsonicTypes';
 import type { ServerProfile } from '@/store/authStoreTypes';
-import { songToTrack } from '@/utils/playback/songToTrack';
+import { songToTrack } from '@/features/playback/utils/playback/songToTrack';
 import { activateShareSearchServer } from '@/utils/share/enqueueShareSearchPayload';
 import { sharePayloadTotal, type ShareSearchMatch } from '@/utils/share/shareSearch';
 import type { ShareSearchPreviewState } from '@/features/search/hooks/useShareSearchPreview';

src/features/search/pages/SearchBrowsePage.tsx

@@ -14,7 +14,7 @@ import StarFilterButton from '@/components/StarFilterButton';
 import { tooltipAttrs } from '@/ui/tooltipAttrs';
 import { APP_MAIN_SCROLL_VIEWPORT_ID } from '@/constants/appScroll';
 import { useAuthStore } from '@/store/authStore';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import { isAdvancedSearchLeaveTargetPath } from '@/features/album';
 import {
   isAdvancedSearchPath,