Half a dozen food chains have held piping-hot stock market debuts in the past year to meet a growing appetite for “fast-casual” restaurants catering to younger and more affluent diners willing to pay more for fresher, higher quality fare than they expect to find at traditional fast food places like McDonald’s. Wall Street hopes the new crop of publicly traded eateries will replicate the success of Chipotle Mexican Grill Inc (CMG.N), which has grown to about 1,800 restaurants since its 2006 debut.