P1: AsyncS3Vfs implementation (Phase 3)

- AsyncS3Vfs: spawn_blocking wrapper over S3Vfs
- AsyncS3File: tokio::sync::Mutex for async state
- Add Clone derive to S3Vfs
- All backend methods wrapped with spawn_blocking

Phase 3 complete: AsyncS3Vfs working
Phase 4 pending: AsyncSmbVfs
Phase 5 pending: WebDAV integration

Tests: 293 passed, 0 failed

markbase-core/src/vfs/mod.rs

@@ -10,6 +10,8 @@ pub mod smb_server_backend;
 pub mod util;
 #[cfg(feature = "async-vfs")]
 pub mod async_fs;
+#[cfg(feature = "async-vfs")]
+pub mod async_s3_fs;
 
 use std::path::{Path, PathBuf};
 use std::time::SystemTime;