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/lib.rs

@@ -0,0 +1,52 @@
+//! SMB2/3 file-sharing server with pluggable storage backends.
+//!
+//! See `docs/superpowers/specs/2026-04-27-rust-smb-server-design.md` for the
+//! v1 design. The public API is small on purpose:
+//!
+//! ```no_run
+//! use smb_server::{SmbServer, Share, Access, ShareBackend};
+//! # async fn run<B: ShareBackend>(backend: B) -> Result<(), Box<dyn std::error::Error>> {
+//! SmbServer::builder()
+//!     .listen("0.0.0.0:4445".parse()?)
+//!     .user("alice", "password")
+//!     .share(Share::new("home", backend).user("alice", Access::ReadWrite))
+//!     .build()?
+//!     .serve()
+//!     .await?;
+//! # Ok(()) }
+//! ```
+
+mod backend;
+mod builder;
+pub(crate) mod conn;
+mod dispatch;
+mod error;
+#[cfg(feature = "localfs")]
+mod fs;
+mod handlers;
+pub(crate) mod info_class;
+pub mod ntstatus;
+mod path;
+mod proto;
+mod server;
+mod utils;
+
+pub use backend::{BackendCapabilities, DirEntry, FileInfo, FileTimes, Handle, OpenIntent, OpenOptions, ShareBackend};
+pub use error::SmbError;
+pub use path::SmbPath;
+pub use builder::{Access, Share};
+#[cfg(feature = "localfs")]
+pub use fs::LocalFsBackend;
+pub use proto::auth::ntlm::Identity;
+pub use server::{ConfigHandle, ShareMode, ShutdownHandle, SmbServer};
+
+pub mod wire {
+    pub use crate::proto::header;
+    pub use crate::proto::messages;
+}
+
+#[cfg(test)]
+mod tests {
+    mod dynamic_config;
+    mod memfs;
+}