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/src/schema.sql

@@ -0,0 +1,58 @@
+PRAGMA journal_mode = WAL;
+PRAGMA foreign_keys = ON;
+PRAGMA synchronous = NORMAL;
+
+CREATE TABLE IF NOT EXISTS files (
+    id              INTEGER PRIMARY KEY,
+    origin_id       TEXT NOT NULL,
+    real_path       TEXT NOT NULL,
+    virtual_path    TEXT NOT NULL,
+    
+    title           TEXT,
+    artist          TEXT,
+    album           TEXT,
+    album_artist    TEXT,
+    genre           TEXT,
+    year            INTEGER,
+    track           INTEGER,
+    disc            INTEGER,
+    duration_ms     INTEGER,
+    bitrate         INTEGER,
+    sample_rate     INTEGER,
+    format          TEXT,
+    
+    origin_mtime    INTEGER NOT NULL,
+    origin_size     INTEGER NOT NULL,
+    content_hash    TEXT,
+    chunk_manifest  BLOB,
+    last_sync       INTEGER NOT NULL DEFAULT (strftime('%s', 'now')),
+    
+    UNIQUE(origin_id, real_path)
+);
+
+CREATE TABLE IF NOT EXISTS artwork (
+    id          INTEGER PRIMARY KEY,
+    file_id     INTEGER NOT NULL REFERENCES files(id) ON DELETE CASCADE,
+    art_type    TEXT NOT NULL,
+    chunk_hash  TEXT NOT NULL,
+    width       INTEGER,
+    height      INTEGER,
+    mime_type   TEXT,
+    UNIQUE(file_id, art_type)
+);
+
+CREATE TABLE IF NOT EXISTS collections (
+    id          INTEGER PRIMARY KEY,
+    name        TEXT NOT NULL UNIQUE,
+    query_json  TEXT NOT NULL,
+    created_at  INTEGER NOT NULL DEFAULT (strftime('%s', 'now')),
+    updated_at  INTEGER NOT NULL DEFAULT (strftime('%s', 'now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_files_virtual ON files(virtual_path);
+CREATE INDEX IF NOT EXISTS idx_files_artist_album ON files(artist, album);
+CREATE INDEX IF NOT EXISTS idx_files_content_hash ON files(content_hash);
+CREATE INDEX IF NOT EXISTS idx_files_real ON files(origin_id, real_path);
+CREATE INDEX IF NOT EXISTS idx_files_origin ON files(origin_id);
+CREATE INDEX IF NOT EXISTS idx_files_last_sync ON files(last_sync);
+CREATE INDEX IF NOT EXISTS idx_artwork_file ON artwork(file_id);