fix: split mujoco and motrix backend extras

[TATP-233]

Summary

• split mujoco-uni and motrixsim-core into independent mujoco and motrix optional dependencies
• keep default make setup installing both backends, add setup-mujoco and setup-motrix, and sync the ROCm project/lock to the same dependency layout
• make backend exports lazy so importing Motrix paths does not eagerly require MuJoCo, and add CLI/test coverage for missing-backend diagnostics

Validation

• make test-all