在训练期间,负 delta_indices (历史帧)会从剧情的末尾静默读取

This is a regression. N1.5's `gr00t/data/dataset.py` padded out-of-range history via `retrieve_data_and_pad(..., padding_strategy="first_last")`, which mapped front indices to `array[0]`. The N1.6 release commit 4e62473 (#456) removed that file and introduced the unguarded `iloc` path.