perf: critical Compose recomposition hotspot fixes

[lostf1sh]

Targets the Critical / High items from section 4 (UI/Compose performance) of the multi-agent codebase review.

Stacked on top of PR #2048. This PR's base is `security/xxe-allowlist-ai-keys-fk`. Once #2048 merges, this PR will rebase onto master automatically. The diff visible here is just the perf work.

What changed (10 task buckets)

1. LibraryScreen.kt recomp hotspots

• Folder tab itemsToShow / songsToShow: `.toImmutableList()` now runs INSIDE the remember block; `showPlaylistCards` added to the key set (the previous key list missed `playlistMode`).
• `rememberPagerState` was conditional on `isCompactNavigation` — lost scroll state on toggle. Replaced by a single unconditional call with a mode-aware `initialPage` + `pageCount` lambda.
• Gradient color lists wrapped in `remember(dm, primaryContainer, onPrimaryContainer)` with `persistentListOf` so they don't re-allocate every frame.
• `getSelectionIndex` bound method reference hoisted into a `remember(multiSelectionState)` so the three tabs receive the same lambda identity across recompositions.

2. LibraryMediaTabs.kt

• `getAlbumColorSchemeFlow(uri)` wrapped in `remember(artUri)` — was running synchronization + dispatcher launch on every recomp inside the items lambda.
• Placeholder `MutableStateFlow<ColorSchemePair?>(null)` allocations replaced with a file-scope `EMPTY_ALBUM_COLOR_SCHEME_FLOW`.

3. LibraryPlaybackAwareSongItem N×N flow subscriptions

• Each item used to spin up its own `stablePlayerState.map{}.distinctUntilChanged()` collector. With 100+ visible items that is 100+ upstream subscriptions. Lifted into the parent tabs as a single `LibraryPlaybackHints(currentSongId, isPlaying)`; items receive that one instance.

4. CastBottomSheet.kt

• `stablePlayerState` sliced to just `isPlaying` so position ticks (~4×/s) don't recompose the sheet.
• `availableRoutes` / `bluetoothDevices` / `activeBluetoothName` / `devices` derived lists wrapped in `remember(inputs)`. `activeDevice` deliberately left inline (captures `stringResource`, composable-only).
• The 13 `collectAsStateWithLifecycle` calls were deliberately kept separate — Compose smart-skipping already invalidates only the slice that changed; a single combine slice is not an unambiguous win.

5. QueueBottomSheet item animations (Hallazgo 3 reappeared)

• Six independent `animateDpAsState` / `animateColorAsState` / `animateFloatAsState` calls per queue item consolidated into a single `updateTransition` keyed on a `QueueItemAnimState(isCurrentSong, isDragging, isSwipeTargeted)`. Same pattern already applied to `EnhancedSongListItem`. `dismissIconAlpha` now derives from `revealProgress × animatedFactor` so the continuous `revealProgress` doesn't need its own animation.
• `queue` param signature: `List` → `ImmutableList` (caller already had it as ImmutableList; the downcast was erasing stability).

6. currentSongArtists stability

• `PlayerViewModel.currentSongArtists`: `StateFlow<List>` → `StateFlow<ImmutableList>`. `FullPlayerSlice.currentSongArtists` and `FullPlayerSlicePart1.currentSongArtists` migrated to match.
• Downstream composables — `FullPlayerSongMetadataSection`, `SongMetadataDisplaySection`, `PlayerSongInfo`, `PlayerArtistPickerBottomSheet` — now accept `ImmutableList` so Compose strong-skipping treats the parameter as stable.
• `SongInfoBottomSheetViewModel.resolvedArtists` also migrated for the picker call site.

7. `derivedStateOf` misuses

• `LyricsSheet.kt` had four `remember(state) { derivedStateOf { state.field } }` wrappers on plain captured values. `derivedStateOf` provides State-dependency tracking — without a State read inside, it's an extra State allocation + snapshot read for nothing. Replaced with direct destructuring.
• `FullPlayerContent.kt:resolvedArtistId` same pattern. Replaced with plain `remember(artists, artistId)`.

8. `LyricsSheet` DataStore reads

• Seven separate `context.dataStore.data.map{}` subscriptions (alignment, translation, romanization, animated, blur enabled, blur strength, keep-screen-on) collapsed into a single mapped `Flow` with `distinctUntilChanged`. New file-private `LyricsSheetPrefs` data class + `Preferences.toLyricsSheetPrefs()` helper.
• Removed a duplicate `DisposableEffect` that registered an identical second lifecycle observer for keep-screen-on (merge artifact).

9. `DailyMixSection` allocations

• `headerSongs` / `visibleSongs` called `songs.take(n).toImmutableList()` on every recomposition. Wrapped in `remember(songs)`.

Verification

• `./gradlew :app:testDebugUnitTest`: 302 tests passing, 0 failing. Same baseline as before the perf work — no regressions.
• `./gradlew :app:assembleDebug`: BUILD SUCCESSFUL.

What's intentionally NOT in this PR

• `List` / `List` / `List` parameter migrations on `PlaylistBottomSheet`, `PlaylistArtCollage`, `MultiSelectionBottomSheet`, `AlbumMultiSelectionOptionSheet`, `PlaylistContainer`. Tried and reverted — each has ~10 call sites passing plain `playlist.songs: List`. Flipping the parameter requires either `.toImmutableList()` boilerplate at every site (anti-pattern) or migrating `Playlist.songs` upstream to `ImmutableList`. The second is the proper fix and belongs to a separate task once the source data model is touched.
• `LyricsSheet` architectural violation. The 7 reads are consolidated into one flow but still bypass `UserPreferencesRepository`. Properly routing them through the repository needs new flow exposures there and is its own task.
• `CastBottomSheet` 13 `collectAsStateWithLifecycle` consolidation. Compose smart skipping already invalidates only the slice that changed; a single combine slice was not an unambiguous win.
• `triggerShuffleAllFromTile` behavioral change is in DO NOT MERGE - security: close XXE, Cast LAN exposure, AI key plaintext, song FK constraint #2048, not here. It belongs there because it was the fix for a failing unit test.

Reviewer notes

• The single largest perf win is probably the `LibraryPlaybackAwareSongItem` lift — going from N collectors to 1 across the songs/favorites tabs.
• The `updateTransition` consolidation in `QueueBottomSheet` mirrors the EnhancedSongListItem refactor that was already in master. Worth keeping the pattern consistent.
• `LibraryPlaybackHints` is marked `@Immutable` — `compose_stability.conf` only covers `data.model.**`, so this annotation is needed explicitly.
• The `gradient*Colors` change uses `persistentListOf` instead of `toImmutableList()` because the input is a small literal list and `persistentListOf` avoids an extra wrap step.
• The reverted `PlaylistArtCollage` / `PlaylistBottomSheet` migrations are still tracked as TODO in the codebase — they're called out in the commit message's "What is NOT in this PR" section so the next maintainer doesn't redo the same revert cycle.

[lostf1sh]

@copilot review

[Copilot]

@copilot review

Reviewed at b891e23. No additional code changes are needed from this follow-up. I also checked Actions runs: the only failure on the base branch was an infra runner shutdown in CodeQL (not a code regression), and recent runs are green/in progress. Local :app:testDebugUnitTest could not be rerun here due toolchain download/network limits (foojay HEAD failure), not due test failures.

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