[Mohr-Coulomb] Add corner return for simultaneous tension-shear yielding

[Curiim]

Summary

This PR extracts only the theory-critical changes in Mohr-Coulomb stress integration.

Changes

• Fix yield admissibility tolerance in compute_yield_state (1e-7)
• Refactor compute_stress into:
  • elastic predictor
  • single-surface return
  • corner return (multi-surface)
• Add dedicated helpers:
  • compute_single_surface_return
  • compute_corner_return
  • update_softening_parameters

[bodhinandach]

I'm blocking this for now, but I think we should take a closer look at this, @ningyiw01.