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/smb-server/src/proto/auth.rs

@@ -0,0 +1,11 @@
+//! NTLMv2 server-side authentication and minimal SPNEGO outer envelope.
+//!
+//! See:
+//! * MS-NLMP — NT LAN Manager (NTLM) Authentication Protocol
+//! * MS-SPNG — Simple and Protected GSS-API Negotiation Mechanism
+//!
+//! v1 implements **only** the NTLM (NTLMSSP) mechanism inside SPNEGO.
+//! Kerberos is out of scope (revisit in v0.2).
+
+pub mod ntlm;
+pub mod spnego;