DALLAS — American Airlines and US Airways expected to spend this week cruising toward completion of a merger that would create the world’s largest airline. Instead, they were stunned Tuesday when the federal government and six states sued to block the deal, saying it would hurt competition and cost consumers hundreds of millions of dollars a year in higher fares and extra fees. Antitrust regulators had done little to interfere with other big airline mergers in the past five years, including Delta …