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/Cargo.toml

@@ -15,6 +15,7 @@ thiserror.workspace = true
 serde.workspace = true
 rmp-serde.workspace = true
 image.workspace = true
+lofty = "0.24"
 parking_lot.workspace = true
 chrono.workspace = true