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/Cargo.toml

@@ -37,6 +37,12 @@ sled = "0.34"
 
 # Hashing (per architecture 8.3)
 xxhash-rust = { version = "0.8", features = ["xxh64"] }
+hex = "0.4"
+
+# Audio metadata
+symphonia = { version = "0.5", default-features = false, features = [
+    "aac", "alac", "flac", "mp3", "ogg", "vorbis", "wav"
+] }
 
 # Testing
 tempfile = "3"