HOUSTON — The armed carjacker projected on a large screen threatens to kill you if you don’t give up your keys. Holding a modified gun that emits a beam, you pull the trigger when he draws his weapon, then seconds later fire again at another person who jumps in front with something in his hand. The second person turns out to be a bystander holding a cellphone. This interactive way of illustrating the use of deadly force is part of unusual training that Houston-area grand jurors can receive before they …