feat(cache): implement Id3v2Handler for MP3 metadata synthesis

- Implement all 8 FormatHandler trait methods
- Use lofty 0.24 for ID3v2.4 tag creation/parsing
- Map all 36 AudioMeta fields to ID3v2 frames
- Handle ID3v2 header parsing for audio_start
- Detect ID3v1 tags at EOF for audio_end
- Add 13 comprehensive unit tests
- Fix test-utils AudioMeta construction with ..Default::default()
- All tests pass, LSP diagnostics clean

crates/musicfs-cache/src/lib.rs

@@ -1,6 +1,8 @@
 mod artwork;
 mod db;
 mod eviction;
+mod format_handler;
+mod format_layout;
 mod metadata;
 mod patterns;
 mod prefetch;
@@ -9,6 +11,8 @@ mod tree;
 pub use artwork::{ArtworkCache, ArtworkError, CachedArtwork};
 pub use db::{Database, TrashedFile, TrashedFilter};
 pub use eviction::{EvictionError, EvictionPolicy, LruEviction};
+pub use format_handler::{FormatError, FormatHandler, FormatHandlerRegistry};
+pub use format_layout::FormatLayout;
 pub use metadata::MetadataCache;
 pub use patterns::{AccessContext, AccessPattern, PatternError, PatternStore};
 pub use prefetch::{PrefetchConfig, PrefetchEngine, PrefetchHandle};