SMB Server Phase 2: VFS backend build fix + integration test

- Add VfsFile: Send supertrait for Mutex compatibility
- Fix SmbServerCommand: struct → Subcommand enum with Start variant
- Fix tracing_subscriber::init() → try_init() to avoid panic when
  logger already initialized
- Fix CLI subcommand name: smb-server → smb-start (flatten naming)
- Add #[command(name = "smb-start")] for CLI disambiguation
- Fix unused variable warnings (smb_fs.rs, smb_server_backend.rs)
- Remove unused VfsFile imports (webdav.rs, scp_handler.rs)
- Integration test: Docker smbclient verified (list, upload, read)

vendor/smb2/src/auth/mod.rs

@@ -0,0 +1,15 @@
+//! Authentication mechanisms for SMB2.
+//!
+//! Supports NTLM authentication (MS-NLMP) and Kerberos authentication
+//! (RFC 4120, MS-KILE).
+//!
+//! Most users don't need this module directly -- [`SmbClient`](crate::SmbClient)
+//! handles authentication during [`connect`](crate::connect).
+
+pub(crate) mod der;
+pub mod kerberos;
+pub mod ntlm;
+pub mod spnego;
+
+pub use kerberos::{KerberosAuthenticator, KerberosCredentials};
+pub use ntlm::{NtlmAuthenticator, NtlmCredentials};