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/writer.py

@@ -0,0 +1,50 @@
+"""Storage writer strategies for pipeline output.
+
+The :class:`StoreWriter` abstraction decouples the pipeline from the
+concrete storage format (bin / h5).  The pipeline builds a ``{key:
+List[Tensor]}`` dict and delegates the write to the writer selected
+by ``output.storage_format``.
+"""
+
+import os
+from abc import ABC, abstractmethod
+from typing import Dict, List
+
+import torch
+
+from astrai.dataset.storage import save_bin, save_h5
+from astrai.factory import BaseFactory
+
+
+class StoreWriter(ABC):
+    """Write pre-tokenized tensors to disk in a format-specific way."""
+
+    @abstractmethod
+    def save(
+        self,
+        output_dir: str,
+        domain: str,
+        shard_idx: int,
+        tensors: Dict[str, List[torch.Tensor]],
+    ) -> None: ...
+
+
+class StoreWriterFactory(BaseFactory["StoreWriter"]):
+    @classmethod
+    def _validate_component(cls, component_cls: type):
+        if not issubclass(component_cls, StoreWriter):
+            raise TypeError(f"{component_cls.__name__} must inherit from StoreWriter")
+
+
+@StoreWriterFactory.register("bin")
+class BinWriter(StoreWriter):
+    def save(self, output_dir, domain, shard_idx, tensors):
+        shard_path = os.path.join(output_dir, domain, f"shard_{shard_idx:04d}")
+        save_bin(shard_path, tensors)
+
+
+@StoreWriterFactory.register("h5")
+class H5Writer(StoreWriter):
+    def save(self, output_dir, domain, shard_idx, tensors):
+        chunk_dir = os.path.join(output_dir, domain)
+        save_h5(chunk_dir, f"data_{shard_idx:04d}", tensors)