A Testable First-Harmonic Bias in Two-Path Interference from Discrete Phase Structure

This work presents a minimal, experimentally testable deviation from standard two-path interference arising from an underlying discrete phase structure. At the level of a single interference fringe, the deviation can be written as a first-harmonic term, appearing as a coupled visibility rescaling and phase offset. However, such a term is not directly observable as a unique signal, since it can be fully absorbed into standard sinusoidal fitting. For this reason, the primary prediction is not a fringe-shape deviation, but a transformation-level constraint. Under controlled quarter-phase shifts, the residual quadrature components are predicted to obey a structured Z₄ rotation law. This produces a calibration-invariant signature that cannot be generically reproduced by noise, detector imbalance, or ordinary fitting procedures. A set of null-control conditions is provided to isolate this behavior experimentally, requiring that the signal: is present only under coherent two-path interference, remains stable across repeated measurements, does not track controllable experimental parameters, and satisfies a quadrature-cycling consistency condition under phase shifts. A characteristic magnitude for the effect is estimated under a class of structural assumptions, yielding a candidate scale on the order of 10^-4 with a more strongly suppressed regime near 10^-7. These values lie near the sensitivity threshold of high-precision interferometric measurements. The result is a concrete, falsifiable prediction: a nonzero residual that obeys a Z₄-locked quadrature cycling law under controlled phase shifts. Absence of such a signal at a given sensitivity constrains the corresponding transport regime rather than invalidating the model outright. This document isolates the observable prediction and experimental test independently of the broader theoretical framework from which it is derived. A falsifiable prediction of a calibration-invariant, Z₄-locked quadrature cycling residual in high-precision interference experiments, together with a phase-cycling protocol that distinguishes it from conventional systematics.