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/src/lib.rs

@@ -1 +1,7 @@
-#![allow(dead_code)]
+mod chunks;
+mod reader;
+mod store;
+
+pub use chunks::{ChunkLocation, ChunkRef};
+pub use reader::{ChunkManifest, FileReader, ReaderError};
+pub use store::{CasConfig, CasError, CasStore, DedupStats};