Reduced Q5 Interference Law

T91 establishes the reduced Q5 two-channel interference law after the \(5→3→2\) reduction. The surviving reduced state space is \(C^2\), with crossing channels \(P\) and \(Q\), and transport generated by \(A=iσ_y\), satisfying \(A^2=-I\). Continuous reduced evolution is given by\[ψ(t)=etAψ(0),\]with\[etA={pmatrix}cos t & sin t\\-sin t & cos t{pmatrix}.\] For the initial relative-phase state\[ψ(0)=1{√2}{pmatrix}1\iφ{pmatrix},\]The evolved channel probabilities are\[(P)=12(1+sin(2t)cosφ),\]and\[(Q)=12(1-sin(2t)cosφ).\]The observable channel imbalance is therefore\[(P)-(Q)=sin(2t)cosφ.\] The theorem shows that reduced Q5 transport naturally produces a two-channel interference structure on the reduced crossing plane. The transport amplitude is controlled by accumulated phase \(t=kε\), while the coherence factor is controlled by the relative phase \(φ\). Phase averaging removes the interference contribution:\[cosφ=0(P)=(Q)=12.\]Loss of coherent phase relation, therefore restores equal channel probabilities. T91 converts the earlier transport-generator chain into an explicit observable probability law. Earlier theorems established\[A^2=-I,^k→ etA,μ_k=sin(kε),\]while T91 derives the resulting two-channel interference structure generated by the same reduced transport algebra. The discrete form follows from\[t=kε,ε=1{320},\]giving\[(P)=12(1+sin(2k/320)cosφ).\]The fringe contrast oscillates with transport step number \(k\), with the first maximal contrast near\[k≈80π≈251,\]matching the near-integer phase-maximization structure identified previously in T84. Classification:Solid: matrix exponential, probability law, normalization, and phase averaging.Conditional: identification of the reduced two-channel Q5 state space and the generator chain \(A=iσ_y\) from T37 and T80-T89.Not claimed: direct experimental observability.