After more than a half-century in business, Baskin Robbins closed Saturday night after a New Canaan woman put in for a last paid order—a small peanut-butter-and-chocolate milkshake. Gretchen Russell had entered the Main Street ice cream store with husband Stephen moments before 10 p.m., when beloved longtime owner Anna Valente-Krolikowski locked the door for the final time. “It’s sad,” said Russell, née Gretchen Meyer, a 1978 New Canaan High School graduate who has patronized Baskin Robbins all her life. “It’s sad to see Anna go. It’s sad to see something we’ve known for so long go.