`src/library.rs:464-471` - O(n³) path lookup

[ducks]

Found by lok hunt:

src/library.rs:464-471 - O(n³) path lookup

• track_by_path() iterates all artists → all albums → all tracks
• Called frequently during playback queue operations
• Fix: Build HashMap<PathBuf, &LibraryTrack> index at load time

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Backend: claude