Last Friday was Pi Day — March 14 — and Max Levchin was celebrating Affirm Holdings’ 13th anniversary. He chose that date in 2012 to incorporate his buy-now-pay-later fintech because he’s “a little bit obsessed with mathematical constants,” he explained.So obsessed, indeed, that he delayed proposing to his now-wife until he could find a 3.14 carat diamond for the engagement ring. A math geek since his childhood in Kyiv, he had spent part of last week trying to finish a set of programming puzzles...