Event Closure Beyond Superposition: A Geometric Admissibility Criterion for Photon Detection

This theoretical note develops an event-admissibility criterion for photon detection within Constrained Null Geometry (CNG). The central distinction is between the Born weight of a decohered channel and the geometric admissibility of that channel as a realized event. In the standard post-decoherence reading, diagonal channels are normally treated as directly eligible event outcomes. This paper inserts an additional CNG closure layer: a null channel can become a detected photon event only if its transported boundary data closes into a timelike boundary record. When all decohered channels are closure-admissible, the ordinary Born rule is recovered. When some channels fail closure, the physical event probabilities are restricted to, or dynamically transported into, the closure-admissible event subspace. The note formulates both a conditional projection rule and a stronger trace-preserving closure-flow realization. The latter gives a possible experimental discriminator: weak-channel dominance without coherent interference and without proportional loss of total event rate. This distinguishes CNG event closure from ordinary interference, optical filtering, detector loss, or post-selection. The paper is theoretical. No laboratory claim is made. Its purpose is to formulate a precise post-decoherence event-closure criterion, derive the corresponding mathematical consequences, and identify a measurable discriminator for future optical tests. The accompanying reproducibility supplement contains Python scripts, generated data, summaries, and figures for the numerical closure-discriminator scans used in the paper.