Jackson Square celebrated a milestone Monday with a ribbon-cutting on a $17 million early education center. But the joy was tempered, for at least one speaker, by the need to keep doing what it takes to lift families out of poverty and violence. About 200 residents and dignitaries marked the opening of the Nurtury Learning Lab outdoors under sunny skies. State Rep. Jeffrey Sánchez, D-Jamaica Plain, urged those gathered to remember the tough times poor Boston families must work through. When at full capacity, the new center will serve 175 students and their families.