San Diego Convention Center, July 23rd. Preview Night. The badge that gets you into Comic-Con before it officially opens — Thursday evening, the floor before the weekend crowds arrive, the exhibitors finishing their setups, the specific quality of walking Hall H before the panels fill it. Preview Night exists in a category by itself. The buying is different: the limited exclusives that sell out by Saturday morning are available now, and the people around you understand exactly what that means. The floor moves at a pace that doesn't exist Friday through Sunday — you can cover the exhibit hall and actually see the displays, have conversations with exhibitors, and take …