I met a different vet this evening as Dr Press only works Thursday mornings but the vet who handled him was very kind and gentle with him.
I did not want to walk away and leave him there even after he was already gone himself. But there are not many places to bury a cat in the city so I let them take care of his remains. I took a bit of fur from that soft spot behind his ears; I'll keep that with me until I myself find it necessary to join him.
I still do not understand what happens when they die. His body was warm and I was able to shut his eyes but I sometimes think that the transition from life to back into the elemental swirl that gives rise to us is a process of some sort but not one that it is possible to know. I have a candle burning for him in case light is useful for him. Or for the rest of us.