Vince Gray beamed as he strode down the aisle, trailed by a cluster of aides and constituents hoping for a photo with the mayor, clutching a fork in one hand and a plate of appetizers in the other. “Imagine how many jobs this will create!” he said. The scene last Wednesday was a preview reception at D.C.’s first Costco, the evening before the 154,000-square-foot store officially opened. Giddy Washingtonians, Marylanders, and local politicos availed themselves of the copious free food and gazed admiringly at the megajugs of liquor and electrical appliances stacked five feet high. Arriving almost exclusively by car, visitors put a solid dent in the Shops at Dakota Crossing’s 2,000-spot parking lot, near the heavily trafficked intersection of New York and South Dakota avenues NE.