Add Week 9 Smart Features: collections, artwork, predictive prefetch

Smart Collections (musicfs-search/src/collections.rs):
- CollectionStore with thread-safe Mutex<Connection>
- CollectionQuery enum: Match, DateRange, RecentlyAdded/Played, MostPlayed, Genre, Compound
- Builtin collections for Recently Added, 80s/90s Music

Artwork Extraction & Caching:
- ArtworkExtractor using symphonia Visual (musicfs-metadata)
- ArtworkCache with CAS storage + on-demand resize (musicfs-cache)
- ArtType: Front/Back/Other, ArtSize: Thumbnail/Medium/Full

Predictive Prefetching:
- PatternStore tracks access patterns with sequence prediction
- PrefetchEngine listens to FileAccessed events, prefetches predictions
- PrefetchOps exposes /.prefetch/ virtual directory with status/hints

Oracle review fixes applied:
- CollectionStore uses Mutex for thread safety
- FileAccessed event now includes file_id for canonical correlation
- JSON parse warnings in collection deserialization

130 tests pass (15 new tests added)

musicfs/crates/musicfs-cas/src/fetcher.rs

@@ -129,6 +129,7 @@ impl ContentFetcher {
     pub fn emit_access_event(&self, meta: &FileMeta, offset: u64, size: u32) {
         if let Some(bus) = &self.event_bus {
             bus.publish(Event::FileAccessed {
+                file_id: meta.id,
                 path: meta.virtual_path.clone(),
                 origin_id: meta.real_path.origin_id.clone(),
                 offset,