Buy the rumor, goes the old Wall Street saying, sell the news. And that may have been what happened with Affirm’s stock Thursday evening after the market closed, when investors sent the shares down 11% on the heels of a massive runup into earnings that saw shares close 10% higher in the official trading session. It may be the case, too, that investors wanted to see more torque in the forward guidance, as management said that, per the shareholder letter penned by CEO Max Levchin accompanying fiscal send quar ...