SAN SALVADOR — One of six siblings whose single mother struggled to make ends meet, Carlos was recruited to the Mara Salvatrucha — one of two Central American gangs whose violent tentacles reach from Los Angeles to Lima — when he was just 12. Yet Carlos’ most dangerous challenge may have been his decision to leave gang life behind him, eventually joining Quetzalcoatl, a small nonprofit (named after a Mesoamerican deity) that works to save youngsters from El Salvador’s rampant, senseless violence.