feat: add native Windows hierarchy test harness

internal/client/spool/private_windows.go

@@ -18,11 +18,9 @@ const privateStateSDDL = "D:P(A;;FA;;;SY)(A;;FA;;;BA)"
 func ensurePrivateFile(path string) error {
 	file, err := os.OpenFile(path, os.O_CREATE|os.O_EXCL|os.O_RDWR, 0o600)
 	if err == nil {
-		err = file.Close()
-	} else if !errors.Is(err, os.ErrExist) {
-		return err
+		return file.Close()
 	}
-	if err != nil {
+	if !errors.Is(err, os.ErrExist) {
 		return err
 	}
 	info, err := os.Lstat(path)