One Reciprocal in One Coefficient: Anatomy of a Failed Interface-Tail Certificate at c=13

Version 4 of this record certified an interval-arithmetic bound on the interface between modes 17{:}5000 and modes 5001{:}\infty of the localized pole-free odd form at c=13, and used that bound to close a 16\times16 Schur comparison and conclude coercivity of the form. The interface bound does not follow from the released code. The released certification/tail_bound.py computes the infinite-mode interface contribution with the prefactor 9/64 at the point where the derivation requires (8/3)^2=64/9. The two numbers are reciprocals. Correcting the coefficient multiplies the released tail by 4096/81=50.5679012345679\ldots, the corrected tail is bounded above by 1.641829102700734064 rather than 0.03246781184540026, and both closure routes in v4 lose their positive global gate. Seven v4 statements are withdrawn as certificates: the interface tail U_\infty\le0.03246781184540026; the total interface bound U_{\mathrm{final}}\le1.01738391101161043; the optional complete interface bound U<0.261474802; the post-H_0 operator denominators 1.54726 and 2.30317; the inference from positive 16\times16 comparison pivots to coercivity of the full form; the conclusion that fixed-c=13 pole-free odd coercivity is established; and the Schur separation \delta_{\mathrm{Schur}}\approx0.1611237793846104656, which v4 defines as \lambda_H-U_{\mathrm{final}} and which therefore inherits the defect.