Implement Week 5 CDC & Delta Detection with Oracle fixes

- Add CdcChunker using FastCDC v3 (16KB/64KB/256KB chunks)
- Add DeltaDetector with scan_origin() returning ScannedFile (no FileId assignment)
- Add OriginWatcher with inotify and 200ms debounce using tokio::spawn
- Fix LocalOrigin::read() to loop until all bytes read
- Add read_full() method to Origin trait
- Add mtime field to ChunkManifest
- Update ContentFetcher to use CDC chunking
- Update bandwidth reduction test to assert >90% (NFR-6.4)

Tests: 71 pass (+11 new)

musicfs/crates/musicfs-origins/src/traits.rs

@@ -26,6 +26,9 @@ pub trait Origin: Send + Sync {
 
     async fn read(&self, path: &Path, offset: u64, size: u32) -> Result<Vec<u8>>;
 
+    /// Read entire file content (for CDC chunking of files <4GB)
+    async fn read_full(&self, path: &Path) -> Result<Vec<u8>>;
+
     async fn exists(&self, path: &Path) -> Result<bool>;
 
     async fn health(&self) -> HealthStatus;