feat: parse into a Document that keeps order and comments

Assisted-by: DeepSeek V4.1 Flash

encode.go

@@ -86,6 +86,15 @@ func (e *encoder) encode(v any) error {
 	if err := e.checkCtx(); err != nil {
 		return err
 	}
+	switch x := v.(type) {
+	case *Document:
+		if x == nil {
+			return fmt.Errorf("interpres: cannot marshal nil value")
+		}
+		return fmt.Errorf("interpres: cannot marshal a Document; marshal its Map() to write the values")
+	case Document:
+		return fmt.Errorf("interpres: cannot marshal a Document; marshal its Map() to write the values")
+	}
 	rv := reflect.ValueOf(v)
 	if !rv.IsValid() {
 		return fmt.Errorf("interpres: cannot marshal nil value")