MEXICO CITY — After years of threats and attacks that Candido Rios blamed on a local mayor furious over his reporting, Rios’ home in Mexico’s deadliest state for journalists was encircled by a fence topped with coils of barbed wire and surrounded by a half-dozen surveillance cameras he monitored on screens in the living room. None of that mattered the afternoon that he stopped, just like every other day on his way home from work, at a highway gas station store that is also a community meeting place of sorts.