fix(gui,cli,gcode): 元数据默认累加模式，格式化Unknown→未知

- BuildOptions.metadata_mode 默认 sum（累加预测和耗材用量）
- CLI --metadata-mode 默认 sum
- GUI 设置回退值默认 sum
- format_duration/format_filament 空值返回 未知 而非 Unknown

a1_swap_mod_packer/gui.py

@@ -738,7 +738,7 @@ class MainWindow(QMainWindow):
         self.skip_duplicates_check.setChecked(bool(options.get("skip_duplicates", True)))
         self.output_dir_edit.setText(str(options.get("output_directory", "")))
         self.output_name_edit.setText(str(options.get("output_filename_rule", DEFAULT_OUTPUT_PATTERN)))
-        metadata_mode = options.get("metadata_mode", "source")
+        metadata_mode = options.get("metadata_mode", "sum")
         metadata_index = self.metadata_combo.findData(metadata_mode)
         if metadata_index >= 0:
             self.metadata_combo.setCurrentIndex(metadata_index)