LAS VEGAS—With a destructive barrage of power punches in the waning minutes of a difficult fight, Canelo Álvarez added another achievement to his overflowing list of boxing accomplishments. The Mexican pound-for-pound superstar is the undisputed super middleweight champion of the world. Álvarez became the first four-belt world champion at 168 pounds in boxing history Saturday night, stopping Caleb Plant in the 11th with two dramatic knockdowns. Álvarez (57–1–2, 39 KOs) added Plant’s IBF title to his own WBC, WBA and WBO belts with a steady tactical performance culminating in a display of his vaunted power to finish the previously unbeaten Plant at the MGM Grand Garden Arena. “It was truly a historic night, and I’m so proud to be a part of it,” Álvarez said. Canelo is generally recognized as the top fighter in the world regardless of weight class. After battering the bigger Plant with dozens of punches that …