Get Mike Santoli's Market Memo in your inbox first. SUBSCRIBE HEREThree weeks into a perky rebound that pushed stocks to fresh records, it's time for the market to prove that blessed relief can give way to genuine belief.The comeback since late July, with the S&P 500 up more than 6% in 12 trading days, has been what I call a "Subtraction of All Fears" rally.These worries weighed down the indexes a few weeks ago: That the leading tech platforms would be further punished for their ever-escalating AI investmen ...