Q5 Fringe-Bias Scale from Leakage Weights

T75 develops a model-dependent prediction for the observable Q5 fringe-bias scale arising from leakage into hidden quarter-phase transport classes. Using the weight parameterization\[w_0=1-ε,_1=ε_1,_2=ε_2,_3=ε_3,\]with\[ε_1+ε_2+ε_3=ε1,\]The theorem derives the exact bias relation\[η^2=(ε+ε_2)^2+(ε_3-ε_1)^2.\]The resulting fringe bias is therefore controlled not only by total leakage magnitude but also by the structure of the leakage distribution across the quarter-phase classes. The cosine-sector deviation arises from the overall leakage displacement term \((ε+ε_2)\), while the sine-sector deviation is governed by the antisymmetric imbalance mode \((ε_3-ε_1)\). T75 is structurally important because it converts the earlier qualitative leakage architecture into a quantitative interferometric prediction. Under the Q5 suppression estimates inherited from T57, the theorem predicts a characteristic fringe-bias scale\[ηQ5~10⁻⁴\]for single-bridge leakage, and\[ηQ5~10⁻⁷\]under an additional return-mediated suppression layer. The theorem, therefore, supplies the missing magnitude estimate needed to complete the experimental prediction arc developed through T72-T74: signal form, structural discriminator, detection threshold, and expected signal scale. T75 does not claim a universal numerical prediction or guaranteed observability across interferometer designs. The predicted magnitude depends on the leakage-path architecture, the symmetry properties of the leakage distribution, and the mapping between the structural suppression parameter \(ε\) and experimentally accessible optical leakage modes. The resulting prediction is therefore conditional rather than uniquely derived. Status: solid for the exact leakage-weight decomposition\[η^2=(ε+ε_2)^2+(ε_3-ε_1)^2\]and for the associated order-of-magnitude scaling analysis under the stated assumptions; conditional on the leakage-path architecture, symmetry structure, and suppression mapping inherited from T57; speculative for the existence or observability of any physical Q5 fringe residual prior to experimental detection.