feat(webdav): Enhance and Harden WebDAV Submodule

This commit introduces a simplified high-level API for the WebDAV submodule and fixes a comprehensive set of critical bugs, security vulnerabilities, and spec-compliance issues.

Key enhancements include:
- A new, user-friendly API (`webdav.Serve`, `webdav.Register`) to simplify serving local directories and registering the WebDAV handler.
- An updated example (`examples/webdav/main.go`) demonstrating the new, cleaner API.

Bug fixes and hardening:
- **Data Integrity:** Fixed a data-loss bug in `memFile.Write` where overwriting parts of a file could truncate it.
- **Resource Management:** Resolved a goroutine leak in `MemLock` by adding a `Close()` method and a shutdown mechanism, now properly managed by the `Serve` function.
- **Recursive Deletion:** Implemented correct recursive deletion in `MemFS.RemoveAll` to ensure proper cleanup.
- **Locking:** Fixed a bug in `MemLock.Create` where it did not check for existing locks, preventing multiple locks on the same resource.

webdav/memlock.go

@@ -67,6 +67,13 @@ func (l *MemLock) Create(ctx context.Context, path string, info LockInfo) (strin
 	l.mu.Lock()
 	defer l.mu.Unlock()
 
+	// Check for conflicting locks
+	for _, v := range l.locks {
+		if v.path == path {
+			return "", os.ErrExist
+		}
+	}
+
 	token := make([]byte, 16)
 	if _, err := rand.Read(token); err != nil {
 		return "", err