Refinement Sector for the Evans Depth Law: Whittaker Cancellation and Residual Drift

Research Note 29 in the "Geometry of the Critical Line" programme. Paper 46 derived the leading asymptotic Evans zero law. This note records the first refinement analysis beyond that leading law, testing first-order correction mechanisms only. Three first-order refinement diagnostics are established. First, the first Whittaker correction, governed by the scaled Coulomb parameter κ_W = 3im²/(8k√λ), cancels from the depth sector at order O(λ−1/2). Second, the quadratic transport term produces a logarithmic correction, but only at order O((log λ)/λ). Third, the regularized first imaginary transport term contributes an O(x−1/2) shift to D_m(x), but with coefficient too small to explain the visible finite-x drift in the m=4 sector. The asymptotic limit D_m^(∞) stands unchanged. The observed finite-λ drift is a genuine multi-source refinement problem, not a defect in the leading law. This note does not exclude higher-order or genuinely global corrections. No arithmetic interpretation is claimed. Part of a 46-paper open-access programme on the geometry of the Riemann zeta function's critical line, anchored by the SCT 5-Manifold and the cover equation Φ + eiπ − 1/Φ = 0.