The generation gap is said to be narrowing as more millennials move back in with, and seek advice from, their parents. But in St. Louis, the chasm may be growing for one group of young adults. Two decades ago, Bosnian genocide survivors arrived in St. Louis penniless and ravaged by war. In one generation, they’ve built businesses, bought homes and raised children who are succeeding at high school and college — and assimilation. A new Mustard Seed play, “Bosnian/American: The Dance for Life,”