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-core/Cargo.toml

@@ -6,6 +6,7 @@ edition.workspace = true
 [dependencies]
 thiserror.workspace = true
 serde.workspace = true
+serde_json.workspace = true
 toml.workspace = true
 tokio = { workspace = true, features = ["sync"] }
 xxhash-rust.workspace = true