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/miniPlayer/components/MiniPlayer.tsx

@@ -4,7 +4,7 @@ import { useEffect, useRef, useState } from 'react';
 import { emit } from '@tauri-apps/api/event';
 import { invoke } from '@tauri-apps/api/core';
 import { useTranslation } from 'react-i18next';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import { registerQueueDragHitTest } from '@/lib/dnd/DragDropContext';
 import MiniContextMenu from '@/features/miniPlayer/components/MiniContextMenu';
 import type { MiniSyncPayload, MiniControlAction, MiniTrackInfo } from '@/features/miniPlayer/utils/miniPlayerBridge';

src/features/miniPlayer/components/MiniToolbar.tsx

@@ -4,7 +4,7 @@ import { emit } from '@tauri-apps/api/event';
 import { Blend, Infinity as InfinityIcon, ListMusic, MoveRight, Shuffle, Volume2, VolumeX, Waves } from 'lucide-react';
 import type { TFunction } from 'i18next';
 import type { MiniSyncPayload } from '@/features/miniPlayer/utils/miniPlayerBridge';
-import { getTransitionMode } from '@/utils/playback/playbackTransition';
+import { getTransitionMode } from '@/features/playback/utils/playback/playbackTransition';
 
 interface Props {
   state: MiniSyncPayload;

src/features/miniPlayer/hooks/useMiniQueueDrag.ts

@@ -1,7 +1,7 @@
 import React, { useEffect, useRef, useState } from 'react';
 import { emit } from '@tauri-apps/api/event';
 import { useDragDrop } from '@/lib/dnd/DragDropContext';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 
 interface Args {
   queueOpen: boolean;

src/features/miniPlayer/utils/miniPlayerBridge.ts

@@ -1,11 +1,11 @@
 import { getCurrentWindow } from '@tauri-apps/api/window';
 import { listen, emitTo } from '@tauri-apps/api/event';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import { useAuthStore } from '@/store/authStore';
-import { setTransitionMode, type TransitionMode } from '@/utils/playback/playbackTransition';
+import { setTransitionMode, type TransitionMode } from '@/features/playback/utils/playback/playbackTransition';
 import { resolveQueueTrack } from '@/utils/library/queueTrackView';
 import type { SubsonicOpenArtistRef } from '@/lib/api/subsonicTypes';
-import type { Track } from '@/store/playerStoreTypes';
+import type { Track } from '@/features/playback/store/playerStoreTypes';
 
 export const MINI_WINDOW_LABEL = 'mini';
 

src/features/miniPlayer/utils/miniPlayerHelpers.ts

@@ -1,7 +1,7 @@
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import { resolveQueueTrack } from '@/utils/library/queueTrackView';
 import type { MiniSyncPayload, MiniTrackInfo } from '@/features/miniPlayer/utils/miniPlayerBridge';
-import type { Track } from '@/store/playerStoreTypes';
+import type { Track } from '@/features/playback/store/playerStoreTypes';
 
 /** Half-width of the mini initial-snapshot queue window (matches the bridge). */
 const MINI_SNAPSHOT_HALF = 100;