When the Celtics lost Game 7 of the 2023 Eastern Conference finals to the Heat, first-year coach Joe Mazzulla was at a low point. But he believed that to truly chase greatness, he needed to be crushed. He was texting about it with Josh Waitzkin, a former child chess prodigy whose story was chronicled in the 1993 Oscar-nominated film “Searching for Bobby Fischer.” “He was pissed,” Waitzkin said. “He also knew it was time to take himself on.”