DARIEN, Conn. — In one of the country’s richest towns — where Mercedes, BMWs and Land Rovers cruise tree-lined streets of multimillion-dollar homes — a man who proposed building more accessible housing says angry neighbors took out their frustration on his son: a 9-year-old boy who was demoted to a lower-level Little League team. Christopher Stefanoni says in a federal lawsuit that residents of Darien are so worried that affordable housing will draw black people to town that they’ll do …