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/config/preprocess_config.py

@@ -87,7 +87,7 @@ class OutputConfig(BaseConfig):
     position_ids_mode : Optional[str]
         How to compute position_ids in packed sequences.
 
-        - ``None`` / ``"none"``: do not generate (backward compatible).
+         - ``"none"``: do not generate (default).
         - ``"doc_reset"``: reset to 0 at each document boundary.
         - ``"continuous"``: sequential 0, 1, 2, ... (pretrain, single doc).
     """
@@ -96,7 +96,7 @@ class OutputConfig(BaseConfig):
     storage_format: str = "bin"
     max_tokens_per_shard: int = 100_000_000
     dtype: Dict[str, str] = field(default_factory=dict)
-    position_ids_mode: Optional[str] = None
+    position_ids_mode: str = "none"
 
 
 @dataclass