Add xtb rex module support

[UMI5751]

Summary

• add staging module lock entries for grimme_lab_xtb_* entrypoints
• add raw-config xtb wrappers with Bullet3 defaults
• add module docs and onboarding test fixtures

Testing

• python -m pip install -e .
• python -c "import rush; print(rush.file)"
• python -m compileall -q src/rush
• timeout 1200s python -m pytest -q tests/test_onboarding_xtb_rex.py

Notes

• module repo: https://github.com/talo/xtb_rex @ 50ce82ade05cd50def1bc8373e9f9fdace570eaf
• module keys:
  • grimme_lab_xtb_single_point_rex
  • grimme_lab_xtb_gradient_rex
  • grimme_lab_xtb_optimize_rex
  • grimme_lab_xtb_hessian_rex
  • grimme_lab_xtb_optimized_hessian_rex
  • grimme_lab_xtb_biased_hessian_rex
  • grimme_lab_xtb_md_rex
  • grimme_lab_xtb_metadyn_rex
  • grimme_lab_xtb_optimized_md_rex
  • grimme_lab_xtb_metaopt_rex
  • grimme_lab_xtb_path_rex
  • grimme_lab_xtb_mode_following_rex
  • grimme_lab_xtb_reactor_rex
  • grimme_lab_xtb_dipro_rex
  • grimme_lab_xtb_vip_rex
  • grimme_lab_xtb_vea_rex
  • grimme_lab_xtb_vipea_rex
  • grimme_lab_xtb_vfukui_rex
  • grimme_lab_xtb_vomega_rex
  • grimme_lab_xtb_ceh_rex
  • grimme_lab_xtb_esp_rex
  • grimme_lab_xtb_stm_rex
  • grimme_lab_xtb_raman_rex
  • grimme_lab_xtb_oniom_rex