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/ntstatus.rs

@@ -0,0 +1,41 @@
+//! NTSTATUS constants used by SMB2 handlers.
+//!
+//! Cross-referenced with MS-ERREF §2.3.1 (NTSTATUS Values). Only the codes the
+//! v1 server actually emits or recognizes live here — kept tight on purpose.
+
+pub const STATUS_SUCCESS: u32 = 0x0000_0000;
+pub const STATUS_PENDING: u32 = 0x0000_0103;
+pub const STATUS_NOTIFY_CLEANUP: u32 = 0x0000_010B;
+pub const STATUS_NOTIFY_ENUM_DIR: u32 = 0x0000_010C;
+pub const STATUS_BUFFER_OVERFLOW: u32 = 0x8000_0005;
+pub const STATUS_NO_MORE_FILES: u32 = 0x8000_0006;
+
+pub const STATUS_INVALID_HANDLE: u32 = 0xC000_0008;
+pub const STATUS_INVALID_PARAMETER: u32 = 0xC000_000D;
+pub const STATUS_NO_SUCH_FILE: u32 = 0xC000_000F;
+pub const STATUS_OBJECT_NAME_NOT_FOUND: u32 = 0xC000_000F;
+pub const STATUS_INVALID_DEVICE_REQUEST: u32 = 0xC000_0010;
+pub const STATUS_END_OF_FILE: u32 = 0xC000_0011;
+pub const STATUS_MORE_PROCESSING_REQUIRED: u32 = 0xC000_0016;
+pub const STATUS_ACCESS_DENIED: u32 = 0xC000_0022;
+pub const STATUS_BUFFER_TOO_SMALL: u32 = 0xC000_0023;
+pub const STATUS_OBJECT_NAME_INVALID: u32 = 0xC000_0033;
+pub const STATUS_OBJECT_NAME_COLLISION: u32 = 0xC000_0035;
+pub const STATUS_OBJECT_PATH_NOT_FOUND: u32 = 0xC000_003A;
+pub const STATUS_OBJECT_PATH_SYNTAX_BAD: u32 = 0xC000_003B;
+pub const STATUS_SHARING_VIOLATION: u32 = 0xC000_0043;
+pub const STATUS_DELETE_PENDING: u32 = 0xC000_0056;
+pub const STATUS_LOGON_FAILURE: u32 = 0xC000_006D;
+pub const STATUS_FS_DRIVER_REQUIRED: u32 = 0xC000_019C;
+pub const STATUS_NOT_SUPPORTED: u32 = 0xC000_00BB;
+pub const STATUS_FILE_IS_A_DIRECTORY: u32 = 0xC000_00BA;
+pub const STATUS_NETWORK_NAME_DELETED: u32 = 0xC000_00C9;
+pub const STATUS_BAD_NETWORK_NAME: u32 = 0xC000_00CC;
+pub const STATUS_UNEXPECTED_IO_ERROR: u32 = 0xC000_009C;
+pub const STATUS_DIRECTORY_NOT_EMPTY: u32 = 0xC000_0101;
+pub const STATUS_NOT_A_DIRECTORY: u32 = 0xC000_0103;
+pub const STATUS_USER_SESSION_DELETED: u32 = 0xC000_015C;
+pub const STATUS_INFO_LENGTH_MISMATCH: u32 = 0xC000_0004;
+pub const STATUS_FILE_CLOSED: u32 = 0xC000_0128;
+pub const STATUS_INVALID_INFO_CLASS: u32 = 0xC000_0003;
+pub const STATUS_NO_EAS_ON_FILE: u32 = 0xC000_0052;