Slim PlayerViewModel: extract selection actions and playback dispatch…

[theovilardo]

… core

Pass 6 - multi-selection batch actions -> MultiSelectionStateHolder:

• Move the songs/albums/genres play/queue/play-next trios, like/unlike, ZIP share, and album/genre song resolution out of the ViewModel.
• Deduplicate while moving: the album and genre trios collapse into shared launchAlbumSelectionAction/launchGenreSelectionAction helpers and like/unlike into one updateFavoritesForSelection. Toast strings, ordering, and clearSelection timing preserved verbatim.
• Holder gains MusicRepository + @ApplicationContext ctor deps; ViewModel supplies playback/toast collaborators via SelectionActionCallbacks.
• Remove dead queueAndPlaySelectedAlbums (zero callers).

Pass 7 - playback dispatch core -> new PlaybackDispatchStateHolder:

• Move both showAndPlaySong overloads, library/favorites full-queue playback with its job/token staleness machinery, playSongs, playSongsShuffled, internalPlaySongs, queue-segment batching, song hydration, playExternalUri, the shuffle-all tile entry point, loadAndPlaySong, the preparing-song pill state, and addSongToQueue/addSongNextToQueue.
• @ViewModelScoped holder injects its 13 @singleton deps directly; ViewModel-owned state (controller, UI state, sheet, toasts, transition job) crosses one PlaybackDispatchCallbacks bundle set at init.
• ViewModel ctor: +playbackDispatchStateHolder, -appShortcutManager, -externalMediaStateHolder (both now unused there).
• PlayerViewModelTest constructs the real dispatch holder wired to the same mocks so end-to-end playback tests keep covering the moved logic.

All public ViewModel signatures are preserved as thin delegates, so no UI files change. PlayerViewModel: 5,061 -> 3,962 lines.

Verified: compileDebugKotlin, PlayerViewModelTest, PlayerViewModelHydrationTest, QueueStateHolderTest, PlaybackStateHolderTest.