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/mod.rs

@@ -0,0 +1,16 @@
+//! SMB2/3 wire-format types, framing, signing, and authentication primitives.
+//!
+//! Layered into:
+//! * [`framing`] — Direct-TCP/NetBIOS transport framing.
+//! * [`header`]  — SMB2 64-byte fixed header.
+//! * [`messages`] — Per-command request/response structs.
+//! * [`auth`]    — NTLMv2 server-side authentication and minimal SPNEGO.
+//! * [`crypto`]  — Signing, key derivation, pre-auth integrity.
+//! * [`error`]   — Crate-wide error type.
+
+pub mod auth;
+pub mod crypto;
+pub mod error;
+pub mod framing;
+pub mod header;
+pub mod messages;