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-core/src/types.rs

@@ -132,6 +132,30 @@ pub struct AudioMeta {
     pub bitrate: Option<u32>,
     pub sample_rate: Option<u32>,
     pub format: AudioFormat,
+    pub track_total: Option<u32>,
+    pub disc_total: Option<u32>,
+    pub date: Option<String>,
+    pub composer: Option<String>,
+    pub comment: Option<String>,
+    pub lyrics: Option<String>,
+    pub copyright: Option<String>,
+    pub compilation: Option<bool>,
+    pub artist_sort: Option<String>,
+    pub album_artist_sort: Option<String>,
+    pub album_sort: Option<String>,
+    pub title_sort: Option<String>,
+    pub mb_recording_id: Option<String>,
+    pub mb_album_id: Option<String>,
+    pub mb_artist_id: Option<String>,
+    pub mb_album_artist_id: Option<String>,
+    pub mb_release_group_id: Option<String>,
+    pub replaygain_track_gain: Option<f32>,
+    pub replaygain_track_peak: Option<f32>,
+    pub replaygain_album_gain: Option<f32>,
+    pub replaygain_album_peak: Option<f32>,
+    pub channels: Option<u32>,
+    pub bits_per_sample: Option<u32>,
+    pub encoder: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]