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/ioctl.rs

@@ -0,0 +1,59 @@
+//! IOCTL handler — handles FSCTL_VALIDATE_NEGOTIATE_INFO; everything else
+//! returns NOT_SUPPORTED.
+
+use std::sync::Arc;
+
+use crate::proto::header::Smb2Header;
+use crate::proto::messages::{Fsctl, IoctlRequest, IoctlResponse};
+
+use crate::conn::state::Connection;
+use crate::dispatch::HandlerResponse;
+use crate::handlers::negotiate::{NEGOTIATE_CAPABILITIES, NEGOTIATE_SECURITY_MODE};
+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 IoctlRequest::parse(body) {
+        Ok(r) => r,
+        Err(_) => return HandlerResponse::err(ntstatus::STATUS_INVALID_PARAMETER),
+    };
+
+    match req.fsctl() {
+        Fsctl::ValidateNegotiateInfo => {
+            // Build VALIDATE_NEGOTIATE_INFO_RESPONSE per MS-SMB2 §2.2.32.6:
+            // Capabilities (4) | Guid (16) | SecurityMode (2) | Dialect (2) = 24 bytes.
+            let dialect = conn.dialect.read().await.map(|d| d.as_u16()).unwrap_or(0);
+            let mut out = Vec::with_capacity(24);
+            out.extend_from_slice(&NEGOTIATE_CAPABILITIES.to_le_bytes());
+            out.extend_from_slice(server.config.server_guid.as_bytes());
+            out.extend_from_slice(&NEGOTIATE_SECURITY_MODE.to_le_bytes());
+            out.extend_from_slice(&dialect.to_le_bytes());
+
+            let resp = IoctlResponse {
+                structure_size: 49,
+                reserved: 0,
+                ctl_code: req.ctl_code,
+                file_id: req.file_id,
+                input_offset: 0,
+                input_count: 0,
+                output_offset: 0x70,
+                output_count: out.len() as u32,
+                flags: 0,
+                reserved2: 0,
+                output: out,
+            };
+            let mut buf = Vec::new();
+            resp.write_to(&mut buf).expect("IOCTL response encodes");
+            HandlerResponse::ok(buf)
+        }
+        Fsctl::DfsGetReferrals | Fsctl::DfsGetReferralsEx => {
+            HandlerResponse::err(ntstatus::STATUS_FS_DRIVER_REQUIRED)
+        }
+        _ => HandlerResponse::err(ntstatus::STATUS_NOT_SUPPORTED),
+    }
+}