feat(robot): add headless JSON contract export

[omnilink-tech]

Summary

• Add robot_contract(robot), a JSON-serializable description built from BulletLab's named Robot/Joint/Link APIs.
• Record the action joint order and state-vector layout alongside joint limits and link masses.
• Add a DIRECT-mode example that prints the contract or writes it to a requested JSON file.
• Cover the schema against the existing Kuka fixture and document the new example.

I left runtime-specific body IDs out of the exported contract so the result remains useful across separate headless runs.

Validation

• pytest tests/test_robot_contract.py tests/test_robot.py tests/test_headless_smoke.py (32 passed)
• Ran examples/10_headless_robot_contract.py against kuka_iiwa/model.urdf and parsed the generated JSON (7 actions, 27 state values).
• python -m compileall -q bulletlab/robot/contract.py examples/10_headless_robot_contract.py tests/test_robot_contract.py
• git diff --check