Boston police identified Thursday the teenager who was shot Monday night on Sanford Street in Mattapan. Arthur Stewart, 19, was shot at about 8:30 p.m. in the area of 57 Sanford St. and was pronounced dead at Boston Medical Center, police said in a statement. Stewart was slain about two hours after another 19-year-old, identified by relatives as Jorge Fuentes, was fatally shot across the street from his home in Dorchester. No arrests have been made in either case.