Good news is that the engine is fine. That seems to mean that my accelerator cable has slipped. In the meantime, I'm having a hard time finding the gas pedal, so when I got to the light, I looked down to see what had happened.
It was gone.
The entire gas pedal had flipped up and under the dashboard. The light changed green, I let the vehicle idle through the intersection and pulled over to check it out.
The gas pedal bracket had broken loose from the firewall. I actually pulled the whole thing out and was standing on the side of the road holding the gas pedal in my hand.
I managed to relocate it in such a way that by pushing my foot sideways, I was able to drive, and I made it home.
Happily, the hardware store and some uncomfortably hard work later means that the gas pedal is quite firmly reattached and the car is driveable. Total cost of the repair: 60 cents. And about 2 hours of squirming underneath the car in impossible positions.
Tomorrow I will be in pain, but for now I feel like something worked in this bizarre world.