Susquehanna International Group makes its money buying and selling stocks in milliseconds‚ eking out fractions of a penny each time. At one millisecond at the end of last year‚ its holdings included a 2% stake in a blank-check company that this week merged with Donald Trump’s social-media company. That has fed theories that Susquehanna’s founder‚ Republican mega-donor Jeff Yass‚ is funneling money to a cash-strapped Trump by pumping up shares of a company that began trading today under the...