Implement Week 2 metadata extraction and cache database

Week 1 fixes:
- Move hex to workspace dependencies
- Add cargo-criterion, protobuf, grpcurl to flake.nix

Week 2 implementation:
- musicfs-metadata: MetadataParser with symphonia 0.5 for FLAC, MP3,
  Opus/Vorbis, M4A/AAC (2 tests)
- musicfs-cache: SQLite schema per architecture 4.3.6 with track/disc
  columns, TEXT content_hash, all required indexes
- musicfs-cache/db.rs: Database with upsert, CRUD, mtime lookup (9 tests)
- musicfs-cache/metadata.rs: MetadataCache with store/lookup/is_fresh/
  invalidate (2 tests)
- musicfs-core: Added Metadata error variant

22 tests pass total. Oracle-verified against architecture doc.

musicfs/flake.nix

@@ -33,6 +33,11 @@
             # Dev tools
             cargo-watch
             cargo-nextest
+            cargo-criterion
+            
+            # gRPC tooling (Week 10+)
+            protobuf
+            grpcurl
           ];
           
           RUST_BACKTRACE = "1";