The AMetric Boundary as a Unique Regime-Invariant Constraint on Admissibility-Bearing Construction

We construct the AMetric boundary as a necessary structural constraint on admissible construction over fixed domains. The construction starts from pre-fixational non-fixation, relabeling invariance, and the exclusion of admissibility-relevant asymmetry without witness. It shows that no selector, grading, ordering, metric, probability, typicality rule, canonical representative, or discrete index can carry admissibility-bearing authority at the boundary. The main contribution is a fixed-domain closure theorem for the AMetric interface. Samescope variation is locked to a comparison class preserving boundary typing, evaluated fragment, standing/admissibility classification, lawful quotient, finite-history continuation structure, and anchor–tensor–skin role carrier. Within that class, constructional behavior is exhausted by two loci: evaluated-act classification and coherent retained-trajectory structure. Any purported boundary variation must enter one of those loci; if it changes neither, it is bookkeeping, and if it changes the comparison class, it is scope change. This local closure spine yields the apex result. Under intelligibility, irreversible commitment, unrestricted composition, scope discipline, and meta-invariance of admissibility criteria (MIAX), together with the constructed binary and non-eventful AMetric boundary, the framework determines: gate existence and bivalent standing on the evaluated fragment; prohibition of admissibility-relevant parameterization, including discrete indexing of multiple admissible alternatives; no same-act repair, no generators, and no primitive standing-carriers; no crossdomain standing transfer without independent target gating and target domain-of-application closure; target-fixation commutation as a condition for theory-level admissible witness transfer; and uniqueness of the admissible interior. A governance-necessity theorem records the same constraint in state-transition form: target fixation, standing, and irreversible commitment require fail-closed admissibility gating in every non-degenerate construction regime inside the comparison class. Operator closure is then formulated with partial algebras. Operator domains encode admissibility, envelope-maximality fixes old domains, and no admissible extension totalizes a partial operator on the original carrier. Conservative admissibility-preserving extensions therefore collapse to definitional bookkeeping or exit the original scope. The exhaustion schema is applied to the mathematical operator spectrum and to the physical-description spectrum, with auxiliary obstruction results for symmetry-based canonical selection and locality-preserving theory extension. The final closure statement is scope-specific: the manuscript does not classify all formal systems; it classifies all admissibility-bearing variation within the fixed comparison class defined herein. The paper also records the conditional export role of the AMetric boundary for downstream admissibility-bearing regimes. All constructed objects are internal to the typed-domain framework, and no empirical predictions are introduced.