She hasn't been checked for parasites yet. Her poop looks normal except that it is soft/liquidy. No specks, chunks, or mucus. Smooth and normal colour.
She was on a chicken-based kibble before I switched. And the evening wet treat was usually Felidae or Innova or EVO mixed with some kitty tuna.
The vet said that the congestion is probably a viral infection, and suggested trying lysine. He said that the next step would be a rhinoscopy to check for nasal polyps (requiring anesthesia, obviously), which I don't really want to subject her to if I don't have to. In the past, when one of my cats has had a viral infection responsive to lysine, they have had a runny nose and weepy eyes. Onu has thick snot. When she sneezes it comes flying out of her nose in a big glob, which she proceeds to eat...