refactor(experiment): 重构模型保存机制，支持 processors 持久化

- 模型保存路径改为 models/{model_type}/ 目录结构
- save_model_with_factors 新增 fitted_processors 参数
- 新增 load_processors 函数加载处理器状态
- Storage 查询排序优化：ORDER BY ts_code, trade_date

src/experiment/regression.py

@@ -598,4 +598,5 @@ if SAVE_MODEL:
             model_path=model_save_path,
             selected_factors=SELECTED_FACTORS,
             factor_definitions=FACTOR_DEFINITIONS,
+            fitted_processors=fitted_processors,
         )