Architect Robert A.M. Stern is best known for designing buildings for the wealthiest New Yorkers. Some of the largest residential deals in the U.S. have been at his towers overlooking Manhattan’s Central Park. Now, Mr. Stern’s eponymous firm is putting the finishing touches on the design for a Miami Beach project that is seeking to entice those same buyers to the Sunshine State. The project will be the firm’s first oceanfront residential building.