STONINGTON, CONN. — Marguerite Riordan, who as a leading dealer in American furniture and folk art built top collections, nurtured major collectors and defined taste in her specialty through her long presence at New York’s Winter Show, died Monday, March 18.  Riordan, who was 92, had largely retired after auctioning her personal collection at Christie’s in 2008. She is survived by her husband of nearly 70 years, Arthur, and their children Victoria and Arthur.  No service is planned. A look at Riordan’s accomplishments and contributions is planned for a coming issue of Antiques and The Arts Weekly.