CDN-354: The Precision Ladder

CDN-354 converts the CDN-352/353 uncertainty ledger into a measurement-design ladder. With the gate margin S = Theta - T_required(C,R) fixed, a covariance-shaped external fit becomes decisive only when |S|/sigma_S clears the chosen threshold. The strong anchor is already a 2-sigma closure under the independent moderate profile, but all-positive covariance requires a sharper score uncertainty to resolve the same cap vector. The locked result is DECISIVENESS_IS_MARGIN_OVER_ERROR_NOT_LABEL.