refactor : pipeline 策略化拆分，消除 _flush if/else

- PackingStrategy / PositionIdStrategy / StoreWriter 独立文件 + Factory
- Pipeline._flush 零 if/else，纯编排
- SectionRenderer 从 SectionedMaskBuilder 分离
- OutputConfig.position_ids_mode 默认改为 ""none""

astrai/preprocessing/__init__.py

@@ -3,12 +3,30 @@ from astrai.preprocessing.builder import (
     MaskBuilderFactory,
     SectionedMaskBuilder,
 )
+from astrai.preprocessing.packing import (
+    PackingStrategy,
+    PackingStrategyFactory,
+)
 from astrai.preprocessing.pipeline import Pipeline, filter_by_length
+from astrai.preprocessing.position_id import (
+    PositionIdStrategy,
+    PositionIdStrategyFactory,
+)
+from astrai.preprocessing.writer import (
+    StoreWriter,
+    StoreWriterFactory,
+)
 
 __all__ = [
     "BaseMaskBuilder",
     "MaskBuilderFactory",
-    "SectionedMaskBuilder",
+    "PackingStrategy",
+    "PackingStrategyFactory",
     "Pipeline",
+    "PositionIdStrategy",
+    "PositionIdStrategyFactory",
+    "SectionedMaskBuilder",
+    "StoreWriter",
+    "StoreWriterFactory",
     "filter_by_length",
 ]