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-core/Cargo.toml

@@ -8,4 +8,4 @@ thiserror.workspace = true
 serde.workspace = true
 tokio = { workspace = true, features = ["sync"] }
 xxhash-rust.workspace = true
-hex = "0.4"
+hex.workspace = true

musicfs/crates/musicfs-core/src/error.rs

@@ -17,6 +17,9 @@ pub enum Error {
     #[error("Cache error: {0}")]
     Cache(String),
 
+    #[error("Metadata extraction error: {0}")]
+    Metadata(String),
+
     #[error("Database error: {0}")]
     Database(String),