Phase 5: WebDAV async integration design framework

- Detailed design notes for AsyncVfsDavFs
- AsyncVfsDavFile implementation pattern
- DavFileSystem trait async implementation
- Estimated: ~3 hours for full implementation

Phase 5 framework documented for future implementation

markbase-core/src/vfs/mod.rs

@@ -625,3 +625,55 @@ pub trait AsyncVfsBackend: Send + Sync {
 // - WebDAV integration: 3 hours
 // - Tests: 2 hours
 // Total: ~13 hours (multi-day project)
+//
+// ===== Phase 5 WebDAV Async Integration Design =====
+//
+// 现状分析：
+// 1. dav-server DavFileSystem trait 方法返回 Pin<Box<dyn Future>>
+// 2. 当前 VfsDavFs::open() 返回 Box::pin(ready(...))
+// 3. 这是 "同步包装为 Future"，不是真正的 async
+//
+// Phase 5 目标：
+// 1. AsyncVfsDavFs 使用 AsyncVfsBackend
+// 2. open() 调用 AsyncVfsBackend::open_file().await
+// 3. read_dir() 调用 AsyncVfsBackend::read_dir().await
+//
+// 实现步骤：
+// 1. 创建 AsyncVfsDavFs 结构体
+// 2. 创建 AsyncVfsDavFile 结构体（包装 AsyncVfsFile）
+// 3. 实现 DavFileSystem trait（调用 AsyncVfsBackend）
+//
+// 关键代码模式：
+// ```rust
+// impl DavFileSystem for AsyncVfsDavFs {
+//     fn open<'a>(&'a self, path: &'a DavPath, options: OpenOptions) -> FsFuture<'a, Box<dyn DavFile>> {
+//         Box::pin(async move {
+//             let vfs_path = self.resolve_path(path)?;
+//             let flags = OpenFlags::from_open_options(&options);
+//             let file = AsyncVfsBackend::open_file(&*self.vfs, &vfs_path, &flags).await?;
+//             Ok(Box::new(AsyncVfsDavFile::new(file)) as Box<dyn DavFile>)
+//         })
+//     }
+// }
+// ```
+//
+// AsyncVfsDavFile 实现：
+// ```rust
+// pub struct AsyncVfsDavFile {
+//     inner: Box<dyn AsyncVfsFile>,
+//     position: u64,
+// }
+//
+// impl DavFile for AsyncVfsDavFile {
+//     fn read_bytes(&mut self, count: usize) -> FsFuture<'_, Bytes> {
+//         Box::pin(async move {
+//             let mut buf = vec![0u8; count];
+//             let n = AsyncVfsFile::read(&mut self.inner, &mut buf).await?;
+//             buf.truncate(n);
+//             Ok(Bytes::from(buf))
+//         })
+//     }
+// }
+// ```
+//
+// 预估工作量：Phase 5 ~3 hours