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/orbit/components/OrbitDiagnosticsPopover.tsx

@@ -3,7 +3,7 @@ import { createPortal } from 'react-dom';
 import { Copy, Trash2 } from 'lucide-react';
 import { useTranslation } from 'react-i18next';
 import { useOrbitStore } from '@/features/orbit/store/orbitStore';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import { showToast } from '@/utils/ui/toast';
 import { computeOrbitDriftMs } from '@/features/orbit/utils/orbit';
 import {

src/features/orbit/components/OrbitSessionBar.tsx

@@ -1,11 +1,11 @@
 import { getSong } from '@/lib/api/subsonicLibrary';
-import { songToTrack } from '@/utils/playback/songToTrack';
+import { songToTrack } from '@/features/playback/utils/playback/songToTrack';
 import { useEffect, useRef, useState } from 'react';
 import { X, RefreshCw, Shuffle, Settings2, Share2, HelpCircle, Activity } from 'lucide-react';
 import { useTranslation } from 'react-i18next';
 import { useOrbitStore } from '@/features/orbit/store/orbitStore';
 import { useHelpModalStore } from '@/store/helpModalStore';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import {
   endOrbitSession,
   leaveOrbitSession,

src/features/orbit/components/OrbitStartModal.tsx

@@ -12,7 +12,7 @@ import {
 } from '@/features/orbit/utils/orbit';
 import { randomOrbitSessionName } from '@/features/orbit/utils/orbitNames';
 import { useAuthStore } from '@/store/authStore';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import { isLanUrl, serverShareBaseUrl } from '@/utils/server/serverEndpoint';
 import { ORBIT_DEFAULT_MAX_USERS } from '@/features/orbit/api/orbit';
 

src/features/orbit/hooks/useOrbitGuest.ts

@@ -1,9 +1,9 @@
 import { getSong } from '@/lib/api/subsonicLibrary';
-import { songToTrack } from '@/utils/playback/songToTrack';
+import { songToTrack } from '@/features/playback/utils/playback/songToTrack';
 import { useEffect, useRef } from 'react';
 import { useOrbitStore } from '@/features/orbit/store/orbitStore';
 import { useAuthStore } from '@/store/authStore';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import {
   readOrbitState,
   applyOrbitTransitionSettings,

src/features/orbit/hooks/useOrbitHost.ts

@@ -1,8 +1,8 @@
 import { getSong } from '@/lib/api/subsonicLibrary';
-import { songToTrack } from '@/utils/playback/songToTrack';
+import { songToTrack } from '@/features/playback/utils/playback/songToTrack';
 import { useEffect } from 'react';
 import { useOrbitStore } from '@/features/orbit/store/orbitStore';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import {
   writeOrbitState,
   sweepGuestOutboxes,

src/features/orbit/hooks/usePlaybackRateOrbitSync.ts

@@ -1,6 +1,6 @@
 import { useEffect } from 'react';
 import { useOrbitStore } from '@/features/orbit/store/orbitStore';
-import { usePlaybackRateStore } from '@/store/playbackRateStore';
+import { usePlaybackRateStore } from '@/features/playback/store/playbackRateStore';
 
 /** Re-sync playback rate when Orbit enters or leaves shared playback. */
 export function usePlaybackRateOrbitSync() {

src/features/orbit/utils/guest.ts

@@ -1,9 +1,9 @@
 import { createPlaylist, deletePlaylist, getPlaylist, getPlaylists, updatePlaylist } from '@/lib/api/subsonicPlaylists';
 import { getSong } from '@/lib/api/subsonicLibrary';
-import { songToTrack } from '@/utils/playback/songToTrack';
+import { songToTrack } from '@/features/playback/utils/playback/songToTrack';
 import { useAuthStore } from '@/store/authStore';
 import { useOrbitStore } from '@/features/orbit/store/orbitStore';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import {
   orbitOutboxPlaylistName,
   type OrbitQueueItem,

src/features/orbit/utils/host.test.ts

@@ -23,10 +23,10 @@ vi.mock('@/features/orbit/utils/remote', () => ({
 }));
 vi.mock('@/features/orbit/store/orbitStore', () => ({ useOrbitStore: { getState: () => orbitStore } }));
 vi.mock('@/store/authStore', () => ({ useAuthStore: { getState: () => ({}) } }));
-vi.mock('@/store/playerStore', () => ({ usePlayerStore: { getState: () => ({ enqueue: vi.fn() }) } }));
+vi.mock('@/features/playback/store/playerStore', () => ({ usePlayerStore: { getState: () => ({ enqueue: vi.fn() }) } }));
 vi.mock('@/lib/api/subsonicPlaylists', () => ({ createPlaylist: vi.fn(), deletePlaylist: vi.fn() }));
 vi.mock('@/lib/api/subsonicLibrary', () => ({ getSong: vi.fn() }));
-vi.mock('@/utils/playback/songToTrack', () => ({ songToTrack: vi.fn() }));
+vi.mock('@/features/playback/utils/playback/songToTrack', () => ({ songToTrack: vi.fn() }));
 
 import { updateOrbitSettings } from '@/features/orbit/utils/host';
 

src/features/orbit/utils/host.ts

@@ -1,9 +1,9 @@
 import { createPlaylist, deletePlaylist } from '@/lib/api/subsonicPlaylists';
 import { getSong } from '@/lib/api/subsonicLibrary';
-import { songToTrack } from '@/utils/playback/songToTrack';
+import { songToTrack } from '@/features/playback/utils/playback/songToTrack';
 import { useAuthStore } from '@/store/authStore';
 import { useOrbitStore } from '@/features/orbit/store/orbitStore';
-import { usePlayerStore } from '@/store/playerStore';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
 import {
   makeInitialOrbitState,
   orbitOutboxPlaylistName,