ARLINGTON, Texas — Scenes from a weekend: Rangers starting pitchers didn’t allow a run in back-to-back games. Aroldis Chapman was acquired, arrived, and blew gas by some Houston hitters. The Rangers packed the All-Star roster like a clown car. Lovely, all of it. The only thing that threatens to ruin the ol’ holiday barbecue is this: By the time the first firework is launched, the Astros may have…