[python] Support ordered batched video frame reads

[XiaoHongbo-Hope]

Purpose

Shuffled and delta-window batches can alternate video payloads and frame ordinals, causing avoidable decoder opens, backward seeks, and duplicate decoding.

Changes

• Decode base and delta rows in one batch.
• Deduplicate identical frame descriptors.
• Group by physical video and decode each group in frame order.
• Restore the requested row order and preserve decoder LRU semantics.

This changes neither the public API nor the .video format. Persisted TorchCodec custom_frame_mappings remains a separate format-level follow-up because current descriptors do not carry that mapping.

Tests

• python -m unittest pypaimon.tests.multimodal_video_test pypaimon.tests.multimodal_lerobot_test
• flake8 --config=dev/cfg.ini on changed files