CDN-190: The Middle Branch Holds

CDN-190 freezes the stable mid-xi four-point response-peak law from CDN-187 and tests a withheld theta60 point. The frozen forecast predicts eps_pk,60 = 0.042833, while the fresh signed response surface observes 0.043927, an error of +0.001094. The mid-xi branch remains monotone through five late-time points, and the five-point fixed-floor residual remains low. Together with CDN-189, this gives the D9 peak-law candidate two independent theta60 holdout passes, while D9 remains unpaid because the floor is still imported.