Once vilified for its stingy health benefits, Wal-Mart has become an unlikely leader in the effort to provide affordable care without bankrupting employers, their workers, or taxpayers in the process. At a time when other firms are scaling back or eliminating health coverage, Wal-Mart has made a serious dent in the problem of the uninsured. New figures show that 5.5% of its employees now lack health insurance, compared with a nationwide rate of 18%.