RIO DE JANEIRO — By this time of the year, rain should be drenching large swaths of the Amazon rainforest. Instead, a punishing drought has kept the rains at bay, creating dry conditions for fires that have engulfed hundreds of square miles of the rainforest that do not usually burn. The fires have turned the end of the dry season in the northern part of the giant rainforest into a crisis. Firefighters have struggled to contain enormous blazes that have sent choking smoke into cities across Sout