refactor: replace diamond inheritance with mixin composition

- StreamStore/RecordStore → Streamable/Recordable (stateless mixins)
- Store is sole base class, no MRO ambiguity
- H5Store/MmapStore/JsonlStore mix in both traits explicitly
- segments_are_records declared per-subclass (H5/Jsonl=True, bin=False)
- Add tests for dpo_tokenize, lazy jsonl, dual-mode H5, stream-only bin
- Remove unused _to_tensor helper

astrai/dataset/__init__.py

@@ -9,10 +9,10 @@ from astrai.dataset.storage import (
     H5Store,
     JsonlStore,
     MmapStore,
-    RecordStore,
+    Recordable,
     Store,
     StoreFactory,
-    StreamStore,
+    Streamable,
     detect_format,
 )
 from astrai.serialization import (
@@ -28,8 +28,8 @@ __all__ = [
     "dpo_collate_fn",
     "grpo_collate_fn",
     "Store",
-    "StreamStore",
-    "RecordStore",
+    "Streamable",
+    "Recordable",
     "StoreFactory",
     "H5Store",
     "MmapStore",