A Threshold Signature of Discrete Quantum Updates in Which-Path Experiments

A testable prediction within the Leibold Unified Field Theory (LUFT) framework: the Englert-Greenberger-Yasin duality relation (V^2 + D^2 <= 1) is not the final bound. LUFT-T1 predicts that in a which-path interferometer with continuously tunable coupling, the visibility-vs-distinguishability curve departs from the EGY quarter-circle at an interior point - strictly below one bit of which-path entanglement entropy - where standard quantum mechanics predicts V > 0. The departure is a threshold snap across a finite margin, preceded by growing run-to-run fringe fluctuations (a second, independent signature). Falsification criterion: if the V-vs-D curve follows the EGY quarter-circle smoothly through the interior at sufficient resolution, the proposal is falsified outright. The threshold S_crit and the reharmonizing rate R are phenomenological parameters, not derived; the work is single-particle and nonrelativistic. Formalization assistance: Lenore (AI collaborator).