Black Holes as Degenerate Closure Boundaries in Constrained Null Geometry

This paper develops the black-hole sector of constrained null geometry as the next step in the constrained-null sequence. Building on the preceding papers, where timelike mass was derived from non-collinear null closure, the reduced two-dimensional fluctuation sector was fixed geometrically, the logarithmic law was shown to be spectrally unavoidable, and the infrared closure scale was determined by the global closure operator, the present work addresses the terminal regime of regular mass realization. A black hole is interpreted not as an ordinary dynamical object, but as a degenerate closure boundary of the null system. Regular massive structure corresponds to a nondegenerate admissible class of constrained null configurations; the black-hole regime appears when that class collapses to a degenerate limit in which outward realizability is lost. Using the completed exterior derivation, the paper identifies the unique static spherically symmetric vacuum exterior with the Schwarzschild metric and interprets the horizon as the first-loss boundary of outward radial null continuation. The black-hole ring is correspondingly reinterpreted as the observational signature of the last outwardly admissible null structures near this degenerate boundary.