Gov. Spencer Cox wants the country to see him as America’s designated grownup, the statesman who rose above our political wreckage to model something better. That is the pitch behind his signature “Disagree Better” campaign and his new book, “Off Ramp: How to Be a Peacemaker in an Age of Contempt,” promoted near the anniversary of Charlie Kirk’s assassination. But the real test of a peacemaker is not how eloquently he condemns contempt in the ab…