After more investigation, it turns out that this was/is standard 1/2" copper that had a solder-lined sleeve over it. The valve and the sleeve were a packaged deal. That's why the walls appeared thicker and that's why nothing fit it to do what I wanted.
The photos in this question show the sleeve quite well. In my case, unfortunately, the end of the sleeve was in the wall and not visible for my initial assessment.
The pipe cutter obscured the two layers in the cross section and likely also added an internal lip that gave me an incorrect inside diameter reading.
I have since been able to get heat and pliers on it and was able to painstakingly remove the sleeve peeling back bit by bit like a bad tin can lid. After peeling back enough of it, I was able to attach a standard compression shut off to the stub.
Soon after adding the compression shut off, I realized that in my heavy heating to de-solder the sleeve as I peeled it back, I may have melted an upstream fitting. Not knowing how far away that might be tucked in the wall, I opened the wall and found a questionable looking upstream elbow joint just behind the drywall.
I replaced the elbow and stub like I should have just done in the first place.