Description: Define the class of grade- and blocklift-stable equivalence-side general
partition-equivalence spans. The equivalence-side analogue of
Pet2Parts : stability of PetErs under one-step grade shift along
SucMap . Ensures that the equivalence-side formulation supports the
same tower/grade infrastructure as the partition-side formulation.
SucMap ShiftStable is the grade axis and does not change the
equivalence-vs-partition viewpoint (reinforced by pets2eq ).
(Contributed by Peter Mazsa, 19-Feb-2026)