Modestly budgeted, the “Purge” series has worked fast — four movies in six years. The new one’s the most violent, but also the least propulsive, with a deliberate, lurching, stop-and-start rhythm and subpar digital photography. Still, it’s notable how “The First Purge” puts its Trump-trolling instincts first, riffing on everything from the Access Hollywood tape to the casting of Patch Darragh as the string-pulling chief of staff. The actor bears a suspicious resemblance to onetime Trump communications director Sean Spicer. Once the Klan-hooded purgers show up, however, practically begging the people of color on screen for a comeuppance, the men sporting little American flag pins on their well-pressed lapels cease to matter much.