refactor(decouple): extract local-playback resolve substrate to store/

Pull findLocalPlaybackUrl/findLocalPlaybackEntry/hasLocalPersistentPlaybackBytes
+ the entry/index-key membership helpers (entryBelongsToServer,
indexKeyBelongsToServer, findFavoriteAutoEntry, hasLocal{Library,FavoriteAuto}Bytes)
out of features/offline/utils/offlineLibraryHelpers into a new core module
store/localPlaybackResolve.ts. The substrate depends only on authStore +
localPlaybackStore + serverIndexKey utils (no useOfflineStore), so the audio
core can resolve on-disk bytes without inverting into @/features/offline.

Repointed the audio-core consumers (crossfadePreload, playbackUrlRouting,
resolvePlaybackUrl, playTrackAction, promoteStreamCache, hotCachePrefetch,
hotCacheStore) + offline-internal callers + favorites hook; moved the two
barrel vi.mocks onto the new leaf module. offlineLibraryHelpers re-imports the
two primitives it still needs internally.

Decouple Step 2a. Removes the local-bytes offline edge from the audio core;
the resolve* media-resolution family (Step 2b) + orbit seam (Step 3) remain.

tsc 0, lint 0, full suite 319/2353 green.

src/features/offline/utils/libraryTierReconcile.ts

@@ -11,7 +11,7 @@ import {
   entryBelongsToServer,
   findLocalPlaybackEntry,
   indexKeyBelongsToServer,
-} from '@/features/offline/utils/offlineLibraryHelpers';
+} from '@/store/localPlaybackResolve';
 
 interface LibraryTrackProbeResult {
   path: string;