It's been a week since I found Summer covered in scours and looking like the end was near. Since then, I've given her dewormer, vitamin B complex, iron, and ProBios. I've also fed her a cup of cooked oatmeal (with a quarter-cup of brown sugar) twice a day. She is perking back up, and her poop is cleaning back up. She still has ok teeth, and she is eating hay and looking like she's chewing cud. At least, I'm assuming that the ProBios helped and that her rumen is working, especially since her poop is looking closer to normal.
It looks like dog doo now. And it's solid, which is a major improvement. It's still not pellets yet, though.
She may not be out of the woods yet, but I can see meadow through the trees. She still looks a mess because I don't want to give her a bath just yet, but she is acting fine - eating hay with reasonable interest, drinking water, squatting to pee, and generally being herself. And if she dies now, I'm willing to blame old age rather than infirmity.