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/crates/musicfs-cas/Cargo.toml

@@ -4,3 +4,18 @@ version.workspace = true
 edition.workspace = true
 
 [dependencies]
+musicfs-core = { path = "../musicfs-core" }
+tokio.workspace = true
+tracing.workspace = true
+serde.workspace = true
+sled.workspace = true
+xxhash-rust.workspace = true
+bytes.workspace = true
+rmp-serde.workspace = true
+hex.workspace = true
+dirs.workspace = true
+thiserror.workspace = true
+
+[dev-dependencies]
+tempfile.workspace = true
+musicfs-cache = { path = "../musicfs-cache" }