MANCHESTER, N.H. — The official toxicology report states that Andrea Cahill’s son died at age 19 from an accidental fentanyl overdose. But more than three years after Tyler Cahill’s death in his childhood bedroom, she doesn’t believe that. It was a poisoning, she said, and there is no question about whom to blame: “the cartels.” Andrea Cahill believes the governments of Mexico and China should be punished for the drug’s flow into the United States. A political independent who nearly always votes