Good night, kitty-kid.
In the end, it was indeed Tenzing's heart. It was likely quietly giving out for some time, just as we'd been told two years ago it would, and what ended up starting it all was a thrown blood clot somewhere around his right shoulder.
As the day went on, fluid started building up in his lungs. He struggled to breathe, even with sedation.
I brought him home this morning hoping we'd get lucky; by early afternoon, I'd called for a final confirmation with the vet tech, and about an hour later, I gave them my answer: this wasn't a salvageable situation. This cat had survived two heart scares and a massive kidney infection. As his owner and guardian it was up to me to admit the obvious: this was a 13.5-year-old cat with a history of health problems, and who was clearly in irrevocable distress.
So I gave him the one gift I had left to give: peace.
He came to me in 1999 as the runt of the litter, a scratchy-meowed thing who his littermate brother would try to overrun, with mixed results:
He always helped me make beds.
I always got help when I tried to use my laptop on a bed.
No ice cream was safe.
He hated when I traveled, and always sniffed my shoes when I got home.
He always let me know when it was time to go to bed.
His love for being a complete nuisance while I sewed was legendary among my friends and my quilt recipients.
I still have Edmund.
I don't have to ask if they knew they were loved. They knew. They always knew.