Thai Grilled Cockles or Clams with Chiles and Limes

River City is a hulking shopping complex on the banks of the Chao Phraya River. But before you dismiss it as a tourist trap, let me tell you that chef-anthropologist Mark Miller comes here to buy Khmer antiques and that yours truly acquired a very handsome seventeenth-century map of Asia at the Old Maps & Prints shop on the fourth floor. Foodwise, the restaurants in the building are about as inviting as airport fast food joints (actually, the food at Bangkok’s amazing new Suvarnabhumi A