I lost a dear friend on Thursday. During an annual physical last year, the vet told me that Jack, our 19-year-old tuxedo cat, was possibly nearing his final months. He started failing steadily last week, so when he lost control of his back legs and collapsed by his litter box, I knew it was time to let him go. En route to the vet’s office Thursday afternoon, I remembered a small poster I’d purchased a few years ago. Cats are angels with fur, it said. Like most angels — animal or human — Jack arrived on the scene exactly when I needed him.  Our only child had gone off to college…