Implement Week 4b Origin-CAS connector for cache-miss handling

- Add ContentFetcher bridging Origin→CAS on cache miss
- Integrate fetcher into FileReader via with_fetcher() constructor
- Add get_or_fetch_manifest() for lazy manifest loading
- Emit FileAccessed events per FR-18.1 via EventBus
- Add 2 integration tests for e2e fetch flow
- Test count: 60 (was 54)

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

@@ -1,7 +1,9 @@
 mod chunks;
+mod fetcher;
 mod reader;
 mod store;
 
 pub use chunks::{ChunkLocation, ChunkRef};
+pub use fetcher::{ContentFetcher, FetchError};
 pub use reader::{ChunkManifest, FileReader, ReaderError};
 pub use store::{CasConfig, CasError, CasStore, DedupStats};