Protected Topology, Boundary, and Domain in Constrained Null Geometry

This paper develops a specific global step within constrained null geometry. Taking the reduced null-triad geometry established in Five Papers on Constrained Null Geometry as given, it identifies a residual compact mode, constructs the associated phase one-form, defines a protected topological datum, studies its constancy along regular paths, and uses first loss of regularity to derive the protected regular class, its admissible boundary, and the canonical domain of the global closure operator. The main claim of the paper is structural rather than interpretive: within a fixed regular protected sector, boundary, domain, operator, spectral scale, and admissible measure class arise from one and the same geometric construction, rather than from externally imposed boundary conditions or independent completion data. In this sense, the paper presents the global layer as a continuation of the same constrained null geometry from which the local reduced sector already arose. The paper is intended as a focused continuation of the global line opened in Five Papers on Constrained Null Geometry, especially the passage from protected topology to admissibility, domain, operator, and realization.