It’s an hour before curtain at “ The Lost Boys ,” and Ali Louis Bourzgui is rolling out a red yoga mat in his dressing room so he can work his core before the matinee starts. As David, the rock god vampire whose bloodlust drives the Broadway musical’s action, Bourzgui struts across the stage, soars through the air, hangs upside down and, at one point, leaps from a bridge into the orchestra pit. “You work muscles you didn’t know you had,” Bourzgui says as he bends upward into an inverted pyramid