- 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