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/settings/components/StorageTab.tsx

@@ -4,7 +4,7 @@ import { invoke } from '@tauri-apps/api/core';
 import { open as openDialog } from '@tauri-apps/plugin-dialog';
 import { Download, FolderOpen, Trash2, X } from 'lucide-react';
 import { useAuthStore } from '@/store/authStore';
-import { countHotCacheTracks } from '@/store/hotCacheStore';
+import { countHotCacheTracks } from '@/features/playback/store/hotCacheStore';
 import { useLocalPlaybackStore } from '@/store/localPlaybackStore';
 import { formatBytes, snapHotCacheMb } from '@/lib/format/formatBytes';
 import SettingsSubSection from '@/features/settings/components/SettingsSubSection';

src/features/settings/components/audio/AudioOutputDeviceSection.tsx

@@ -8,7 +8,7 @@ import { SettingsGroup } from '@/features/settings/components/SettingsGroup';
 import { SettingsToggle } from '@/features/settings/components/SettingsToggle';
 import { useAuthStore } from '@/store/authStore';
 import { useEqStore } from '@/store/eqStore';
-import { buildAudioDeviceSelectOptions } from '@/utils/audio/audioDeviceLabels';
+import { buildAudioDeviceSelectOptions } from '@/features/playback/utils/audio/audioDeviceLabels';
 
 interface Props {
   audioDevices: string[];

src/features/settings/components/audio/PlaybackRateBlock.tsx

@@ -17,8 +17,8 @@ import {
   playbackSpeedStep,
   varispeedSpeedFromSemitones,
   type PlaybackStrategy,
-} from '@/utils/audio/playbackRateHelpers';
-import { usePlaybackRateStore } from '@/store/playbackRateStore';
+} from '@/features/playback/utils/audio/playbackRateHelpers';
+import { usePlaybackRateStore } from '@/features/playback/store/playbackRateStore';
 import { useOrbitStore } from '@/features/orbit';
 import { useAuthStore } from '@/store/authStore';
 import { isOrbitPlaybackSyncActive } from '@/features/orbit';

src/features/settings/components/audio/TrackTransitionsBlock.tsx

@@ -10,7 +10,7 @@ import {
   getTransitionMode,
   setTransitionMode,
   type TransitionMode,
-} from '@/utils/playback/playbackTransition';
+} from '@/features/playback/utils/playback/playbackTransition';
 import { SettingsGroup } from '@/features/settings/components/SettingsGroup';
 import { SettingsToggle } from '@/features/settings/components/SettingsToggle';
 import { SettingsSubCard, SettingsField, SettingsRow, SettingsValue } from '@/features/settings/components/SettingsSubCard';