refactor : Storage 层重构为 Store，移除 Fetcher 中间层，支持多段数据与显式长度

- 合并 BaseStorage + MultiSegmentFetcher + BaseSegmentFetcher 三层为 Store ABC
- Store._data 直接持有 Dict[str, List[Tensor]]，不做强制拼接避免 OOM
- _fetch_key 统一用 bisect 跨段切片，单段多段同一路径
- _length 显式存储（min total across keys），__len__ 返回 O(1)
- MmapStore/H5Store/JSONStore 统一走 _normalize() 注册分段并预计算累积长度
- 所有 I/O 函数 (save_h5/load_h5/json_to_bin 等) 保持不变

astrai/dataset/dataset.py

@@ -8,8 +8,8 @@ from torch import Tensor
 from torch.utils.data import Dataset
 
 from astrai.dataset.storage import (
-    BaseStorage,
-    StorageFactory,
+    Store,
+    StoreFactory,
     detect_format,
 )
 from astrai.factory import BaseFactory
@@ -26,7 +26,7 @@ class BaseDataset(Dataset, ABC):
         super().__init__()
         self.window_size = window_size
         self.stride = stride
-        self.storage: Optional[BaseStorage] = None
+        self.storage: Optional[Store] = None
 
     @property
     def required_keys(self) -> List[str]:
@@ -65,7 +65,7 @@ class BaseDataset(Dataset, ABC):
         """
         if storage_type is None:
             storage_type = detect_format(load_path)
-        self.storage = StorageFactory.create(storage_type)
+        self.storage = StoreFactory.create(storage_type)
         self._load_path = load_path
         self.storage.load(load_path, tokenizer=tokenizer)
         self._validate_keys()