I took my broken bicycle to a bicycle expert friend, who whipped out his portable bikestand and started investigating. After an hour or so, we concluded that most things are fine, including the frame, pedals, headset, bottom bracket, etc. However, the rack will just have to be discarded, the rear wheel will need to be trued, and the derailer tab on the frame will need to be bent back into alignment. I’ll also need a new brake housing and cable, but that’s pretty minor. I think the rear derailer is probably fine, but it’ll need to be verified.
This process has forced me to take a closer look at the damage… which make me even more confused about what happened. For example, the rear rack was severely contorted, despite being made of thick strong aluminum (I can’t bend it all by hand). There’s a white-colored scrape on the side that was not touching the ground, perhaps white transfer from whatever hit it… but it’s hard to be sure. It’s also hard to imagine what that object might have been. It must have been pretty heavy, or moving pretty fast, but that part of the bicycle was on the sidewalk, far from the kerb… and the whole thing was thoroughly locked in place; it’s hard to imagine that it was moved after being hit.
Oh well. Physics often provides a mystery.