I set up a nice little recovery tank for her, with paper towels for greater cleanliness and solid walls to keep out drafts. I was going to take her over to the vet's in that, but she didn't like it at all, and kept jumping to try to get out. I was worried she'd hurt herself doing that, and she was clearly distressed, so she got to ride over and hang out before surgery in the usual little cage.
I shall draw a veil over the actual surgery, because I wasn't there and according to report that part went according to plan. But, as it turns out, Echo is very sensitive to anaesthetic. She didn't wake up properly until some time early this morning; she spent the evening limper than any live rat I've ever seen, and only very gradually gained the ability to twitch. We were pretty worried, but she seems to be out of it now.
She spent the night and today so far in the nice quiet recovery tank. A little earlier, she was scouting around trying to figure out how to escape, so we let her wander around for a while. She wants to be back in the big cage. She seems to have pretty well worn herself out, so she is now hiding in a box in the tank, resting.
 Including the future. I suppose it's technically possible that worse things could happen, but it's really, really unlikely.