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/smb2/.cargo/audit.toml

@@ -0,0 +1,8 @@
+# Ignored advisories for cargo-audit
+
+[advisories]
+ignore = [
+    # Marvin Attack timing sidechannel in `rsa` crate. No fix available.
+    # Only affects benchmarks/smb/ (via sspi -> rsa), not the smb2 crate itself.
+    "RUSTSEC-2023-0071",
+]