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/handlers/logoff.rs

@@ -0,0 +1,28 @@
+//! LOGOFF handler.
+
+use std::sync::Arc;
+
+use crate::proto::header::Smb2Header;
+use crate::proto::messages::LogoffResponse;
+
+use crate::conn::state::Connection;
+use crate::dispatch::HandlerResponse;
+use crate::ntstatus;
+use crate::server::ServerState;
+
+pub async fn handle(
+    _server: &Arc<ServerState>,
+    conn: &Arc<Connection>,
+    hdr: &Smb2Header,
+    _body: &[u8],
+) -> HandlerResponse {
+    if hdr.session_id == 0 {
+        return HandlerResponse::err(ntstatus::STATUS_USER_SESSION_DELETED);
+    }
+    conn.close_session(hdr.session_id).await;
+    let mut buf = Vec::new();
+    LogoffResponse::default()
+        .write_to(&mut buf)
+        .expect("encode");
+    HandlerResponse::ok(buf)
+}