feat: add motrix training support for x2 wall flip

[Lya416]

Fixes #654

Summary

• Register X2WallFlipTracking for the Motrix backend.
• Add the PPO Motrix owner config for x2_wall_flip_tracking.
• Adjust the X2 simple collision XML for Motrix-compatible contact behavior.

Validation

• uv --cache-dir /tmp/uv-cache run pytest tests/config/test_locomotion_params.py::test_ppo_x2_wall_flip_tracking tests/base/backend/test_mujoco_scene_context_visual.py::test_x2_wall_flip_wires_render_only_visual_twin tests/envs/test_env_configs.py::test_x2_wall_flip_tracking_cfg_uses_x2_wall_flip_profile
• Minimal PPO learn smoke for task=x2_wall_flip_tracking/motrix

Notes

• Training artifacts, logs, checkpoints, ONNX files, and videos are not included.

[TATP-233]

是否验证？请贴上训练曲线和视频

[Lya416]

补充验证如下：

• Motrix 训练 run：logs/rsl_rl_ppo/X2WallFlipTracking/2026-06-24_13-39-38_motrix
• 训练状态：completed
• completed iterations：9499
• total env steps：233472000
• final mean reward：66.75257692337036
• best mean reward：69.00383758544922
• playback checkpoint：model_9499.pt
• playback video：6.0s，1280x720，50 FPS

训练曲线：

视频：

https://raw.githubusercontent.com/Lya416/UniLab/validation/motrix-pr-artifacts-20260701/x2_wall_flip_motrix_play_video.mp4

补充说明：曲线和视频放在 fork 的独立验证分支 validation/motrix-pr-artifacts-20260701，不包含在本 PR 的提交内容里。