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/write.rs

@@ -0,0 +1,60 @@
+//! WRITE handler.
+
+use std::sync::Arc;
+
+use crate::proto::header::Smb2Header;
+use crate::proto::messages::{WriteRequest, WriteResponse};
+
+use crate::builder::Access;
+use crate::conn::state::Connection;
+use crate::dispatch::HandlerResponse;
+use crate::handlers::shared::{lookup_open, lookup_session_tree};
+use crate::ntstatus;
+use crate::server::ServerState;
+
+pub async fn handle(
+    _server: &Arc<ServerState>,
+    conn: &Arc<Connection>,
+    hdr: &Smb2Header,
+    body: &[u8],
+) -> HandlerResponse {
+    let req = match WriteRequest::parse(body) {
+        Ok(r) => r,
+        Err(_) => return HandlerResponse::err(ntstatus::STATUS_INVALID_PARAMETER),
+    };
+    let max_write = *conn.max_write_size.read().await;
+    if req.length > max_write {
+        return HandlerResponse::err(ntstatus::STATUS_INVALID_PARAMETER);
+    }
+    let tree_arc = match lookup_session_tree(conn, hdr).await {
+        Ok(t) => t,
+        Err(s) => return HandlerResponse::err(s),
+    };
+    let granted = {
+        let tree = tree_arc.read().await;
+        tree.granted_access
+    };
+    if !matches!(granted, Access::ReadWrite) {
+        return HandlerResponse::err(ntstatus::STATUS_ACCESS_DENIED);
+    }
+    let open_arc = match lookup_open(&tree_arc, req.file_id).await {
+        Some(o) => o,
+        None => return HandlerResponse::err(ntstatus::STATUS_FILE_CLOSED),
+    };
+    let result = {
+        let open = open_arc.read().await;
+        match open.handle.as_ref() {
+            Some(h) => h.write_owned(req.offset, req.data).await,
+            None => return HandlerResponse::err(ntstatus::STATUS_FILE_CLOSED),
+        }
+    };
+    let count = match result {
+        Ok(n) => n,
+        Err(e) => return HandlerResponse::err(e.to_nt_status()),
+    };
+    let mut buf = Vec::new();
+    WriteResponse::new(count)
+        .write_to(&mut buf)
+        .expect("encode");
+    HandlerResponse::ok(buf)
+}