Implement Week 7 Remote Origins with Oracle fixes

- Add credentials.rs with CredentialStore, redacted Debug (session_token shows [REDACTED])
- Add nfs.rs with ESTALE retry using Fn closure, 5s health timeout
- Add smb.rs with ENOTCONN retry handling, 5s health timeout
- Add s3.rs/sftp.rs feature-gated stubs with security documentation
- Add error variants: S3, Sftp, Timeout, Credential, NfsStaleHandle
- Fix delta.rs unused imports

Oracle fixes applied:
- SMB retry_on_disconnect for ENOTCONN (errno 107)
- session_token Debug shows [REDACTED] when Some, None otherwise
- NFS/SMB health checks wrapped with tokio::time::timeout(5s)

102 tests pass, 0 warnings.

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

@@ -1,14 +1,20 @@
 mod failover;
 mod health;
 mod local;
+mod nfs;
 mod registry;
 mod router;
+mod s3;
+mod sftp;
+mod smb;
 mod traits;
 
 pub use failover::{FailoverExecutor, RetryConfig};
 pub use health::{HealthCheckHandle, HealthMonitor, HealthSnapshot, OriginHealthState};
 pub use local::LocalOrigin;
+pub use musicfs_core::OriginType;
+pub use nfs::NfsOrigin;
 pub use registry::OriginRegistry;
 pub use router::{LatencyStats, Router};
-pub use musicfs_core::OriginType;
+pub use smb::SmbOrigin;
 pub use traits::{Origin, WatchCallback, WatchEvent, WatchHandle};