MEXICO CITY — Amid the constant blare of car horns in southern Mexico City, it’s hard to imagine that Cuicuilco was once the heart of a thriving ancient civilization. Yet atop its circular pyramid, now surrounded by buildings and a shopping center, a pre-Hispanic fire god was revered. “This is incredible,” said Evangelina Báez, who spent a recent morning at Cuicuilco with her daughters. “In the midst of so much urbanization, there’s still this haven of peace.” Her visit was part of a monthly