[python] Add direct LeRobot capture writer

[YannByron]

Summary

Add a LeRobot recording-loop compatible writer that sends captured frames directly to a Paimon multimodal table without materializing an intermediate LeRobot dataset.

Changes

• Add PaimonLeRobotWriter with add_frame, episode save/discard, explicit flush, and finalize lifecycle methods.
• Batch completed episodes into one Paimon commit (10 by default) while keeping episode/global/task indices deterministic.
• Encode image features as PNG BLOBs; MP4/video features remain unsupported in this first version.
• Document the direct-capture API and validate frames, images, schemas, failure states, and tail-batch commits.

Testing

• python3 -m unittest pypaimon.tests.multimodal_lerobot_writer_test pypaimon.tests.multimodal_lerobot_test (56 run, 24 optional LeRobot integration tests skipped)
• python3 -m ruff check pypaimon/multimodal/lerobot/writer.py pypaimon/multimodal/lerobot/loader.py pypaimon/tests/multimodal_lerobot_writer_test.py
• git diff --check

Notes

This draft intentionally starts with PNG image frames, synchronous encoding, and new or empty target tables. MP4 encoding, restart/resume state, and streaming-checkpoint semantics are deferred.