feat: support motrix kp/kd reset randomization

[wlgys8]

Summary

• add Motrix backend reset-lifecycle kp/kd randomization support via PositionActuator overrides
• require all Motrix actuators to be position at backend initialization (fail fast for mixed actuator models)
• cache default actuator kp/kd at backend init, and return cached values in get_actuator_gains()
• expose Motrix DR capabilities for base_mass_delta/base_com_offset/kp/kd plus interval push
• add tests for Motrix randomization behavior (base_com, kp/kd, shape validation, interval push) and manager warning/filter behavior
• update DR user docs to reflect current Motrix capability boundary

Validation

• uv run mypy src
• uv run pytest tests/dr/test_manager.py
• uv run pytest -m slow tests/base/test_sim_backend.py -k "TestMotrixBasic and (randomization or capabilities or interval)"
• uv run pytest tests/scripts/test_check_docs.py

Related

• partially addresses Sim: Motrix 域随机化 #94
• related follow-up contract issue: Work: actuator-type-aware domain randomization contract for mixed actuators #324