Alexander Wang, the precocious designer chosen to head the venerable Balenciaga, did not disappoint his legions of admirers at New York Fashion Week, presenting a sophisticated and occasionally humorous collection Saturday. In the sumptuous Cunard Building down in Manhattan's financial district, the 29-year-old Californian designer of Taiwanese descent sent out about 40 models in single or two-color outfits. The music on the runway was "Eye of the Tiger," of "Rocky" boxing film fame, but instead of Sylvester Stallone's boxing gloves, there were huge black fur mittens.