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-cache/src/eviction.rs

@@ -0,0 +1,155 @@
+use musicfs_cas::CasStore;
+use musicfs_core::ChunkHash;
+use std::collections::BTreeMap;
+use std::sync::RwLock;
+use std::time::Instant;
+use tracing::info;
+
+pub trait EvictionPolicy: Send + Sync {
+    fn record_access(&self, hash: ChunkHash);
+    fn select_victims(&self, count: usize) -> Vec<ChunkHash>;
+    fn remove(&self, hash: &ChunkHash);
+}
+
+pub struct LruEviction {
+    access_times: RwLock<BTreeMap<Instant, ChunkHash>>,
+    hash_to_time: RwLock<std::collections::HashMap<ChunkHash, Instant>>,
+}
+
+impl LruEviction {
+    pub fn new() -> Self {
+        Self {
+            access_times: RwLock::new(BTreeMap::new()),
+            hash_to_time: RwLock::new(std::collections::HashMap::new()),
+        }
+    }
+
+    pub async fn evict_to_target(
+        &self,
+        store: &CasStore,
+        target_size: u64,
+    ) -> Result<u64, EvictionError> {
+        let mut bytes_freed = 0u64;
+
+        while store.current_size() > target_size {
+            let victims = self.select_victims(10);
+
+            if victims.is_empty() {
+                break;
+            }
+
+            for hash in victims {
+                if let Ok(data) = store.get(&hash).await {
+                    bytes_freed += data.len() as u64;
+                    store.delete(&hash).await?;
+                    self.remove(&hash);
+                }
+            }
+        }
+
+        if bytes_freed > 0 {
+            info!("Evicted {} bytes from cache", bytes_freed);
+        }
+
+        Ok(bytes_freed)
+    }
+}
+
+impl Default for LruEviction {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl EvictionPolicy for LruEviction {
+    fn record_access(&self, hash: ChunkHash) {
+        let now = Instant::now();
+        let mut times = self.access_times.write().unwrap();
+        let mut h2t = self.hash_to_time.write().unwrap();
+
+        if let Some(old_time) = h2t.remove(&hash) {
+            times.remove(&old_time);
+        }
+
+        times.insert(now, hash);
+        h2t.insert(hash, now);
+    }
+
+    fn select_victims(&self, count: usize) -> Vec<ChunkHash> {
+        let times = self.access_times.read().unwrap();
+        times.values().take(count).copied().collect()
+    }
+
+    fn remove(&self, hash: &ChunkHash) {
+        let mut times = self.access_times.write().unwrap();
+        let mut h2t = self.hash_to_time.write().unwrap();
+
+        if let Some(time) = h2t.remove(hash) {
+            times.remove(&time);
+        }
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum EvictionError {
+    #[error("CAS error: {0}")]
+    Cas(#[from] musicfs_cas::CasError),
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_lru_access_order() {
+        let lru = LruEviction::new();
+
+        let h1 = ChunkHash::from_bytes(b"chunk1");
+        let h2 = ChunkHash::from_bytes(b"chunk2");
+        let h3 = ChunkHash::from_bytes(b"chunk3");
+
+        lru.record_access(h1);
+        std::thread::sleep(std::time::Duration::from_millis(1));
+        lru.record_access(h2);
+        std::thread::sleep(std::time::Duration::from_millis(1));
+        lru.record_access(h3);
+
+        let victims = lru.select_victims(2);
+        assert_eq!(victims.len(), 2);
+        assert_eq!(victims[0], h1);
+        assert_eq!(victims[1], h2);
+    }
+
+    #[test]
+    fn test_lru_reaccess_updates_order() {
+        let lru = LruEviction::new();
+
+        let h1 = ChunkHash::from_bytes(b"chunk1");
+        let h2 = ChunkHash::from_bytes(b"chunk2");
+
+        lru.record_access(h1);
+        std::thread::sleep(std::time::Duration::from_millis(1));
+        lru.record_access(h2);
+        std::thread::sleep(std::time::Duration::from_millis(1));
+        lru.record_access(h1);
+
+        let victims = lru.select_victims(1);
+        assert_eq!(victims[0], h2);
+    }
+
+    #[test]
+    fn test_lru_remove() {
+        let lru = LruEviction::new();
+
+        let h1 = ChunkHash::from_bytes(b"chunk1");
+        let h2 = ChunkHash::from_bytes(b"chunk2");
+
+        lru.record_access(h1);
+        lru.record_access(h2);
+        lru.remove(&h1);
+
+        let victims = lru.select_victims(10);
+        assert_eq!(victims.len(), 1);
+        assert_eq!(victims[0], h2);
+    }
+}

musicfs/crates/musicfs-cache/src/lib.rs

@@ -1,8 +1,10 @@
 mod db;
+mod eviction;
 mod metadata;
 mod tree;
 
 pub use db::Database;
+pub use eviction::{EvictionError, EvictionPolicy, LruEviction};
 pub use metadata::MetadataCache;
 pub use tree::{
     DirNode, FileNode, Inode, RefreshPolicy, TreeBuilder, VirtualNode, VirtualTree, ROOT_INODE,