“The rule in question concerns the Patient Protection and Affordable Care Act’s tax credits, not the law’s tax increases. The tax credits are intended to offset the cost of insurance premiums for low- and middle-income workers. For many Americans, however, those tax credits are like an anchor disguised as a life vest. The mere fact that a taxpayer is eligible for a tax credit can trigger tax liabilities against both the taxpayer (under the act’s ‘individual mandate’) and her employer (under the ’employer mandate’).”