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/__init__.py

@@ -4,13 +4,11 @@ from astrai.dataset.dataset import (
 )
 from astrai.dataset.sampler import ResumableDistributedSampler
 from astrai.dataset.storage import (
-    BaseSegmentFetcher,
-    BaseStorage,
-    H5Storage,
-    JSONStorage,
-    MmapStorage,
-    MultiSegmentFetcher,
-    StorageFactory,
+    H5Store,
+    JSONStore,
+    MmapStore,
+    Store,
+    StoreFactory,
     detect_format,
     json_to_bin,
     load_bin,
@@ -24,13 +22,11 @@ from astrai.dataset.storage import (
 __all__ = [
     "BaseDataset",
     "DatasetFactory",
-    "BaseSegmentFetcher",
-    "MultiSegmentFetcher",
-    "BaseStorage",
-    "H5Storage",
-    "JSONStorage",
-    "MmapStorage",
-    "StorageFactory",
+    "Store",
+    "StoreFactory",
+    "H5Store",
+    "JSONStore",
+    "MmapStore",
     "detect_format",
     "save_h5",
     "load_h5",