MgMn B2 probe — e_hull degeneracy test (elemental tie-line case)

Degeneracy probe for the MP phase-diagram route: compound input at a composition with (expected) no stable MP compound. If the Mg-Mn hull has no compound competition, the route must return e_above_hull == formation_energy_per_atom exactly (hull reduces to the elemental tie-line), demonstrating the definitional equality case on a compound input. Parsed Pm-3m, 2 atoms, a=3.00 Å, min dist 2.598 Å.