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/Cargo.lock

@@ -687,6 +687,7 @@ version = "0.1.0"
 dependencies = [
  "hex",
  "serde",
+ "serde_json",
  "tempfile",
  "thiserror",
  "tokio",
@@ -727,8 +728,10 @@ version = "0.1.0"
 dependencies = [
  "async-trait",
  "dashmap",
+ "libc",
  "musicfs-core",
  "tempfile",
+ "thiserror",
  "tokio",
  "tracing",
 ]