As their fellow union members did across southern New England, at 1 p.m., employees at the two Stop & Shop supermarkets in Darien went on strike, primarily for better health and retirement benefits. Negotiations with the company hadn’t resulted in progress, the employees at the Heights Road supermarket said. The contract expired in February. The strike that began Thursday was the first one against the chain in this area in 30 years, they said. The parking lot at Stop & Shop on Heights Road had fewer vehicles in it than usual, although the supermarket was still open at about 3:30 p.m., with at least one register still open.