A quiet Monday night in Cancún turned tense when witnesses said a man beat a dog to death in front of his children. Neighbors moved first. They confronted him, blocked his escape, and held him for the police. Authorities with the city’s animal welfare unit later opened a case. The scene, caught in fragments on phones and shared across local groups, touched a nerve in a city already on edge about animal cruelty. Here’s what happened, what official…