feat(cache): implement FlacHandler for FLAC metadata synthesis

- Implement all 8 FormatHandler trait methods
- Parse FLAC metadata blocks and extract STREAMINFO
- Preserve original STREAMINFO in synthesized headers
- Map all 36 AudioMeta fields to Vorbis comment tags
- Binary serialization of Vorbis comments with little-endian lengths
- Add 16 comprehensive unit tests including STREAMINFO preservation
- All tests pass, LSP diagnostics clean

crates/musicfs-cache/src/lib.rs

@@ -3,6 +3,7 @@ mod db;
 mod eviction;
 mod format_handler;
 mod format_layout;
+pub mod handlers;
 mod metadata;
 mod patterns;
 mod prefetch;
@@ -13,6 +14,7 @@ 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 handlers::{FlacHandler, Id3v2Handler};
 pub use metadata::MetadataCache;
 pub use patterns::{AccessContext, AccessPattern, PatternError, PatternStore};
 pub use prefetch::{PrefetchConfig, PrefetchEngine, PrefetchHandle};