AUSTIN — Amber and Adam Briggle don’t talk to their children about their family’s emergency exit plan. The preparations are made quietly, in the background of daily life: The car is kept up-to-date on maintenance, always with a full tank of gas. The carriers are ready for the cats. Amber and Adam know the shortest route across the Texas border, and they know where they’d stay for the short term, while they figured out the family’s next step. Lawyers are poised to tell them when it’s time to leave, should it come to that.