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/mod.rs

@@ -0,0 +1,64 @@
+//! Per-command handlers.
+//!
+//! Each function here builds a `HandlerResponse` for a specific SMB2 command.
+//! Handlers receive the parsed request header and a slice of the body bytes;
+//! they return either a successful body or `HandlerResponse::err(ntstatus)`.
+
+use std::sync::Arc;
+
+use crate::proto::header::{Command, Smb2Header};
+
+use crate::conn::state::Connection;
+use crate::dispatch::HandlerResponse;
+use crate::ntstatus;
+use crate::server::ServerState;
+
+mod change_notify;
+mod close;
+mod create;
+mod echo;
+mod flush;
+mod ioctl;
+mod lock;
+mod logoff;
+pub(crate) mod negotiate;
+mod oplock_break;
+mod query_directory;
+mod query_info;
+mod read;
+mod session_setup;
+mod set_info;
+pub(crate) mod shared;
+mod tree_connect;
+mod tree_disconnect;
+mod write;
+
+/// Top-level command router.
+pub async fn dispatch_command(
+    server: &Arc<ServerState>,
+    conn: &Arc<Connection>,
+    hdr: &Smb2Header,
+    body: &[u8],
+) -> HandlerResponse {
+    match hdr.command {
+        Command::Negotiate => negotiate::handle(server, conn, hdr, body).await,
+        Command::SessionSetup => session_setup::handle(server, conn, hdr, body).await,
+        Command::Logoff => logoff::handle(server, conn, hdr, body).await,
+        Command::TreeConnect => tree_connect::handle(server, conn, hdr, body).await,
+        Command::TreeDisconnect => tree_disconnect::handle(server, conn, hdr, body).await,
+        Command::Create => create::handle(server, conn, hdr, body).await,
+        Command::Close => close::handle(server, conn, hdr, body).await,
+        Command::Flush => flush::handle(server, conn, hdr, body).await,
+        Command::Read => read::handle(server, conn, hdr, body).await,
+        Command::Write => write::handle(server, conn, hdr, body).await,
+        Command::Lock => lock::handle(server, conn, hdr, body).await,
+        Command::Ioctl => ioctl::handle(server, conn, hdr, body).await,
+        Command::Echo => echo::handle(server, conn, hdr, body).await,
+        Command::QueryDirectory => query_directory::handle(server, conn, hdr, body).await,
+        Command::ChangeNotify => change_notify::handle(server, conn, hdr, body).await,
+        Command::QueryInfo => query_info::handle(server, conn, hdr, body).await,
+        Command::SetInfo => set_info::handle(server, conn, hdr, body).await,
+        Command::OplockBreak => oplock_break::handle(server, conn, hdr, body).await,
+        Command::Cancel => HandlerResponse::err(ntstatus::STATUS_INVALID_PARAMETER),
+    }
+}