Fe bcc element probe — e_hull degeneracy test (definitional equality case)

Degeneracy probe for the MP phase-diagram route: elemental input. For an element, formation energy and e_above_hull are definitionally equal (both = E_orb(element) − E_DFT(elemental reference)), so this tests whether the route's e_hull != e_formation intake gate would false-positive on honest elemental rows, and whether negative e_hull is possible when Orb v3 energy differs in sign from the DFT reference. Parsed Im-3m, 2 atoms, a=2.866 Å, min dist 2.482 Å.