A Selection Rule for Magnetoelectric Order in YBCO Planar Oxygen NMR

We define a data-processing invariant in planar oxygen NMR that is protected by partner-exchange symmetry and therefore cancels the conventional spin-fluctuation baseline by construction. For planar oxygen partners in YBCO, we define the in-plane relaxation anisotropy ratio and a representation invariant D(T). We argue that a magnetoelectric transduction channel generically breaks the partner-exchange protection because the local electric field differs between sites. The prediction is a symmetry bifurcation at T*; above T* the partner constraint holds, while below T* the curves split with opposite trends, providing a robust test for loop-current order.