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-core/src/types.rs

@@ -66,9 +66,29 @@ impl ChunkHash {
         Self(xxh64(data, 0).to_le_bytes())
     }
 
-    pub fn to_hex(&self) -> String {
+    pub fn as_hex(&self) -> String {
         hex::encode(self.0)
     }
+
+    pub fn to_hex(&self) -> String {
+        self.as_hex()
+    }
+
+    pub fn from_hex(s: &str) -> Option<Self> {
+        let bytes = hex::decode(s).ok()?;
+        if bytes.len() != 8 {
+            return None;
+        }
+        let mut arr = [0u8; 8];
+        arr.copy_from_slice(&bytes);
+        Some(Self(arr))
+    }
+}
+
+impl std::fmt::Display for ChunkHash {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.as_hex())
+    }
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]