“The presence of externalities and other market imperfections does not justify a departure from the normal rules of the constitutional road. Health care is typically consumed locally, and health-insurance markets themselves primarily operate within the states. The administration’s attempt to fashion a singular, universal solution is not necessary to deal with the variegated issues arising in these markets. States have taken the lead in past reform efforts. They should be an integral part of improving the functioning of health-care and health-insurance markets.”