refactor: split Store into StreamStore and RecordStore

- StreamStore: fetch(begin, end, key) for stream access (SEQ/SFT)
- RecordStore: mixin with fetch_record(i, key) for record access
- H5Store/MmapStore/JsonlStore now dual-inherit both (C3 MRO)
- JsonlStore supports lazy mode via processor= (no TokenizeTransform)
- RecordDataset base class holds processor, DPO/GRPO simplified
- dpo_tokenize pure function for on-the-fly JSONL tokenisation
- DatasetFactory builds processor for jsonl+record datasets
- train.py passes tokenizer_path=param_path uniformly
- progress: len(dataset) returns sample count (stream=windows, record=records)
- json no longer auto-detected as jsonl format

astrai/dataset/__init__.py

@@ -9,8 +9,10 @@ from astrai.dataset.storage import (
     H5Store,
     JsonlStore,
     MmapStore,
+    RecordStore,
     Store,
     StoreFactory,
+    StreamStore,
     detect_format,
 )
 from astrai.serialization import (
@@ -26,6 +28,8 @@ __all__ = [
     "dpo_collate_fn",
     "grpo_collate_fn",
     "Store",
+    "StreamStore",
+    "RecordStore",
     "StoreFactory",
     "H5Store",
     "MmapStore",