KANSAS CITY, Mo. — The Chiefs’ Super Bowl rally was wrapping up Wednesday, and it had been a great day for Chris Cotten and his 13-year-old son, Zach. They had cheered for the reigning Super Bowl champion Chiefs and weren’t far from the Union Station stage. Zach got autographs from a handful of former players. Then there was gunfire. “The police ran from behind the stage towards the shooter, and when they cleared, we went into the building,” Chris Cotten recalled in a phone interview. “Nobody was really actually sure if it was gunshots or if it was fireworks.” Cotten estimated there were a few…