[错误报告] Reacher 观察时将后步 qpos 与过时的身体位置混合

Describe the bug

ReacherEnv._get_obs() appears to construct a single observation from quantities corresponding to different points of the MuJoCo simulation step. After mj_step, the joint angles are read from the updated data.qpos, while the fingertip position is obtained from MuJoCo's derived body-position data. Since _step_mujoco_simulation() does not call mj_forward after mj_step, these quantities can be slightly inconsistent. In our tests, this produces a forward-kinematics residual of approximately 6.2e-9. Calling mj_forward after mj_step reduces the residual to approximately 2.8e-17, near machine precision. The result is that the Reacher observation does not exactly satisfy its expected forward-kinematics relationship when the joints are moving.