Shares of AST SpaceMobile rose as much as 100% in Friday trading to roughly $50, extending their advance to about 300% over the past 12 months. The surge briefly lifted the satellite-telecom start-up’s market value to almost $10 billion, even though the company has yet to generate revenue, according to commentary from research firm Citron. The sharp move attracted brisk activity among retail-oriented traders, adding to a wave of momentum-driven buying that has fueled the stock’s month-long rally.