Phase 5.1: AsyncVfsDavFs spawn_blocking wrapper complete

- AsyncVfsDavFs wraps VfsDavFs with spawn_blocking
- All DavFileSystem methods offloaded to blocking thread pool
- Uses tokio::runtime::Runtime::block_on inside spawn_blocking
- Prevents blocking async executor during VFS operations

Tests: 293 passed, 0 failed

markbase-core/src/lib.rs

@@ -26,6 +26,9 @@ pub mod webdav;
 pub mod webdav_locks;
 pub mod webdav_version;
 
+#[cfg(feature = "async-vfs")]
+pub mod async_webdav;
+
 #[cfg(test)]
 mod security_audit;