AST SpaceMobile just took a significant step toward viability. But it's not the final step yet.Yesterday's stock market sell-off hurt a lot of stocks, but AST SpaceMobile (ASTS 5.65%) wasn't one of them. Defying the market rout, shares of the start-up space company promising satellite internet to any off-the-shelf cellphone all 'round the world rose 0.4% Monday -- after announcing some positive FCC news. AST shares seemed likely to continue rising today, gaining nearly 5% in early trading. But then AST stoc ...