P1: AsyncSmbVfs implementation (Phase 4)

- AsyncSmbVfs: spawn_blocking wrapper over SmbVfs
- AsyncSmbFile: tokio::sync::Mutex for async state
- Add Clone derive to SmbVfs (Arc<Mutex<Tree>>)
- Remove manual Clone impl (derive handles it)

Phase 4 complete: AsyncSmbVfs working
Phase 5 pending: WebDAV integration

Tests: 293 passed, 0 failed

markbase-core/src/vfs/mod.rs

@@ -12,6 +12,8 @@ pub mod util;
 pub mod async_fs;
 #[cfg(feature = "async-vfs")]
 pub mod async_s3_fs;
+#[cfg(feature = "async-vfs")]
+pub mod async_smb_fs;
 
 use std::path::{Path, PathBuf};
 use std::time::SystemTime;