Null-Control Criterion for Q5 Quadrature Cycling

T79 completes the Q5 interferometric falsification framework by introducing a structured null-control protocol for quadrature-cycling residuals. Earlier theorems established the residual form, the \ (Z₄\) quadrature-cycling structure, statistical thresholds, magnitude estimates, and systematics discrimination criteria. T79 addresses the final degeneracy problem: the cycling defect\ Iₐ₅=₊=₀^3|v₊+₁-Jvₖ|² vanish trivially when all quadrature vectors are zero. The theorem, therefore, establishes the joint operational requirement\ Iₐ₅0|vₖ|0, that the detected structure corresponds to nontrivial quadrature transport rather than the simple absence of signal. The theorem then defines four explicit null-control experiments designed to isolate ordinary instrumental artifacts from candidate Q5-consistent cycling structure. These include blocked-path operation, detector-only measurements, intentional laser-power modulation, and a classical phase-reference channel. Each control specifies a concrete pass condition under which stable, nonzero quarter-cycle transport behaviour must disappear or fail to persist. In particular, blocked-path operation forces \ (P₁P₂=0\), eliminating the interference residual derived in T72, while detector-only and reference-channel measurements test whether the observed cycling behaviour originates from instrumentation rather than coherent transport structure. T79 reframes the experimental objective as layered falsification rather than direct proof. A candidate signal survives only if coherent two-path interference exhibits nonzero stable quadrature vectors satisfying the quarter-cycle rotation law, while all null controls fail to reproduce the same structured behaviour. The theorem, therefore, converts the earlier invariant framework into a hardened experimental protocol capable of rejecting broad classes of ordinary systematic effects without claiming definitive proof of Q5 transport itself. Passing the protocol establishes only that the observed cycling behaviour survives the identified null controls and remains structurally consistent with the Q5 quadrature-transport prediction. Status: solid for the joint nontriviality condition \ (Iₐ₅0\) together with \ (|vₖ|0\) ; solid for the blocked-path null implied by the T72 interference structure; solid that the four null controls define experimentally testable falsification operations; conditional on whether the listed controls exhaust all relevant systematic classes for a given experimental platform; not a claim that passing the full null-control protocol would uniquely establish Q5 transport physics.