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.
Alexander
d664439746