I neglected to take photos of this, so it is not going to be a particularly interesting post.
I started by pulling each warm-up injector (there are two on this car) and testing them both with the manual injector switch on the dash. Neither were actuating, even when I cranked it.
Eventually, I noticed that the cold-start relay was unplugged from 12V power. Even though I could hear it actuating, the injectors weren't actually getting a signal. The 12V wire was not attached to the battery particularly well, and it looks like I knocked it loose when I was getting to the oil filter. I diagnosed this as something that the oil change caused, but it looks like it was operator error. What a relief to have it fixed.