Stivi knows about death. The married, father of four has about him the air of mysterious tough guy. IRVINE, California – When the shooter burst through the door, the students were ready. They jumped over and around tables. They rushed the gunman. They screamed and tossed anything within reach; backpacks, books, pens. In less than five seconds, the would-be killer was on the floor, powerless to carry out his planned – and staged – massacre. “How many shots did you get off?” the guest instructor asked after the simulated attack in a University of California, Irvine, classroom. “Two,” the shooter...