Artwork was stored inline in the tracks table (385MB / 98% of DB size for 8K tracks).
Now read directly from audio file metadata via AVAsset when needed, shrinking the DB to ~8MB.
Introduces the core Track struct (nonisolated to work with project-wide
MainActor default isolation), its FetchableRecord/MutablePersistableRecord
conformances, a DEBUG fixture helper, and three Swift Testing tests covering
round-trip persistence, ID assignment on insert, and deterministic hash computation.
