Calibration-Invariant Q5 Signature

T78 establishes the first calibration-invariant experimental signature proposed for Q5-consistent interferometric residuals. Earlier theorems showed that a raw first-harmonic residual of the form\ () =+ not diagnostically meaningful on its own because it can be absorbed into ordinary fitted fringe parameters such as visibility and phase offset. The theorem further demonstrates that simple scalar constructions, including alternating quarter-phase sums, also fail identically for generic first-harmonic signals. T78 therefore shifts the experimental focus away from scalar residual magnitude and toward transformation structure in quadrature space. The central construction is the quadrature-vector formulation. For each controlled quarter-phase offset, the residual is decomposed into quadrature coefficientsₖ= (ₖ, ₖ), the theorem proves that quarter-phase stepping induces the rotation law₊+₁=Jvₖ, \[J=pmatrix0 solid that ideal quadrature-locked cycling implies \ (Iₐ₅=0\) ; conditional as an experimental discriminator because sufficiently engineered systematics could in principle mimic the same cycling behavior; not a claim that observation of \ (Iₐ₅0\) alone would establish Q5 transport physics.