Unity says the clip is playing. I'm looking right at it, and the arm isn't bending. We already fixed one test that could pass with a static clip. Apparently there's another way for this to look like a win in the log.
The clip was addressing bones under paths that missed the imported skin. Codex helped check the paths and the motion together. Once they matched, I could finally see the arm bend.
Then I turned the corrective on and off in Play mode. A single shape missed part of the bend in the middle. More samples got closer to what I authored in Blender, and the rig file got bigger.
I am satisfied with what I can see on this arm today. I still have to find out what happens with two poses, transitions and a whole character.
I can give Codex a pretty specific job now. Get this correction into Unity and give me a way to see whether it survived. The moment I can toggle it myself is when the work starts to feel real.