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/db.rs

@@ -203,6 +203,7 @@ impl Database {
                         bitrate: row.get(13)?,
                         sample_rate: row.get(14)?,
                         format,
+                        ..Default::default()
                     }),
                     size: row.get::<_, i64>(17)? as u64,
                     mtime: UNIX_EPOCH + Duration::from_secs(row.get::<_, i64>(16)? as u64),