Pointwise validity without structural robustness: exact margins for a sign-vector existence certificate

Sign-vector conditions provide sufficient certificates for the existence of a unique positive equilibrium in classes of generalized-polynomial systems. Such a certificate is pointwise in the exponent configuration and does not itself quantify its structural stability. We show that pointwise validity can coexist with zero structural robustness margin of the sign-vector configuration: the configuration in Deshpande–Müller Example 22 satisfies the sufficient certificate yet lies at zero distance from the certificate-failure set. We express this margin as an exact cone-constrained principal-angle distance from the exponent subspace to the corresponding forbidden incidence set. For the same source example, the zero margin is removable, and an explicit nearby rational configuration has a strictly positive structural margin that is globally certified over the finite maximal-cone family and available in closed form. A contrasting partial-minor regime serves as a control case, where the corresponding structural radius reduces to a nearest-wall singular-value law, while a kinetic-deficiency-zero pullback supplies the physical interpretation under admissible species/kinetic-order perturbations. Thus satisfaction of the sign-vector certificate does not by itself imply positive structural robustness of that certificate. A reproduction package accompanies this deposit; all decisive quantities are exact (rational/algebraic with certified real-root isolation), with floating-point solvers used only as numerical corroboration.