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/crates/musicfs-cache/Cargo.toml

@@ -4,3 +4,14 @@ version.workspace = true
 edition.workspace = true
 
 [dependencies]
+musicfs-core = { path = "../musicfs-core" }
+rusqlite = { workspace = true, features = ["bundled"] }
+sled.workspace = true
+tokio.workspace = true
+tracing.workspace = true
+thiserror.workspace = true
+serde.workspace = true
+rmp-serde.workspace = true
+
+[dev-dependencies]
+tempfile.workspace = true