DALLAS — A Texas investor group is planning for a doomsday scenario by building a $300 million luxury community replete with underground homes and air-lock blast doors designed for people worried about a dirty bomb or other disaster. The Trident Lakes community has begun with a flourish northeast of Dallas near the Oklahoma border: A statue of Poseidon, the Greek god of the sea, holding a golden trident will stand some 50 feet above a massive fountain billed as one of the largest in the world. Subtlety won’t do for Texas. “The initial perception is that it’s defined as a doomsday scenario,” said James O’Connor, CEO of Dallas-based Vintuary Holdings, which represents the collection of investors backing the project.