Admissibility Selectivity and Constraint Erosion

This paper isolates admissibility selectivity as a structural invariant of admissible regions and introduces constraint erosion as a monotone transformation acting on admissibility structure. Admissibility selectivity is defined in terms of the symmetry properties of admissible regions, independent of trajectories, mechanisms, or system realisation. Constraint erosion is defined as a strict reduction of admissibility selectivity under preserved admissibility. No assumptions are made regarding dynamics, equilibrium, optimisation principles, attractors, temporal evolution, or mechanisms of constraint enforcement.