Implement Week 4 CAS store with chunk deduplication and LRU eviction

- Add musicfs-cas crate: CasStore, ChunkHash, FileReader, ChunkManifest
- Add LruEviction policy to musicfs-cache for cache size management
- Integrate FileReader into FUSE filesystem for actual file reads
- Use xxHash64 for content hashing, sled for index, msgpack serialization
- Default cache path: ~/.cache/musicfs/chunks/ with 256 subdirs sharding
- 20 new tests (14 CAS unit + 3 integration + 3 eviction), 54 total

musicfs/Cargo.toml

@@ -44,5 +44,11 @@ symphonia = { version = "0.5", default-features = false, features = [
     "aac", "alac", "flac", "mp3", "ogg", "vorbis", "wav"
 ] }
 
+# Bytes handling
+bytes = "1"
+
+# Platform directories
+dirs = "5"
+
 # Testing
 tempfile = "3"