Just above where the San Marcos River tumbles down a small dam several miles east of Interstate 35, the crumbling redbrick hull of an old cotton gin presides over the eastern shore, and a grove of pecan trees shades a wide meadow to the west. It’s as pretty a spot as you’ll find in Central Texas, with year-round 72-degree water and layers of history. Off to the side of the cotton gin, a glint of shiny metal reveals a silver Blue Bird bus parked semipermanently next to a shipping container and a couple of lawn chairs, where a man in shorts rolls a cigarette and leans back to tell his story.During the eighties and nineties, Robert Jenkins was a pioneering manufacturer and distributor of the club…