feat: add record-mode to Store for DPO/GRPO

- Store gains fetch_record/num_records alongside stream fetch/__len__
- save_bin/load_bin support per-record offsets via record_keys param
- H5Store/MmapStore/JsonlStore all support dual stream+record access
- DPODataset/GRPODataset use fetch_record, no cross-record concat
- dpo_collate_fn + collate_fn wired through TrainConfig
- fixes attention context leakage in DPO from windowed concatenation

astrai/serialization/__init__.py

@@ -19,6 +19,7 @@ from astrai.serialization.checkpoint import (
 )
 from astrai.serialization.dataset import (
     load_bin,
+    load_bin_offsets,
     load_h5,
     save_bin,
     save_h5,
@@ -37,6 +38,7 @@ __all__ = [
     "save_safetensors",
     "save_torch",
     "load_bin",
+    "load_bin_offsets",
     "load_h5",
     "save_bin",
     "save_h5",