@lunarloony After this, mine now includes "give fluids immediately, drip-feeding by syringe if necessary".
It didn't occur to me that a cat I found in the rain would be so dehydrated.
(It's pure speculation, but I think she got trapped in the barely-used church I found her outside a door of, was found and put out sometime on Monday night, and by then was too weak to drink from puddles.)