RIO DE JANEIRO — Brazil currently accounts for one-quarter of the entire world’s daily COVID-19 deaths, far more than any other single nation, and health experts are warning the nation is on the verge of even greater calamity. There is growing recognition among experts, mayors and governors that shutdowns are no longer avoidable. Restrictions on activity they implemented last year were half-hearted and consistently sabotaged by President Jair Bolsonaro. He remains unconvinced of any need for clampdown, which leaves local leaders pursuing a patchwork of measures to prevent the death toll from spiraling. It may be too late, with a more contagious variant rampaging across Brazil.