Callum Smith had a front row seat to see his brother dismantled by Canelo Alvarez in a bruising light-middleweight title fight in 2016.Ringside at a raucous AT&T Stadium in Arlington, Texas, Smith can recall in particular the accuracy of Alvarez’s punches, the way he maneuvered his big brother, Liam, into a position to drop him on three occasions — the final time with a brutal body shot.“He finds gaps and holes that you don’t know you’re leaving,” Smith said of the Mexican boxer. “So I knew then, and I know now, how good a fighter he is.”Four years later, it’s time to take some family revenge.Smith will compete in the biggest fight of his career on Saturday when he attempts to do what only one man, the great Floyd Mayweather, has achieved —.