what tool to unbend a simple arc ?

Lazy Nezumi Pro might work, if you are using Windows. It’s scripting can be controlled to give line patterns like below.  The trick is to combine with LNP Ruler presets like Parallel Lines and not move the cursor too quickly.

I think you would need to modify the ‘Square’ script.  You might get some help with that here.

 Guillaume Stordeur is extremely helpful, and would almost certainly give you guidance.

This statement is not true: 

Pshop has not , as far as I can see, added any basic geometry transform tools since its launch in 4.1 days.

There have actually been a number of warp additions since then, such as Perspective Warp. But the solution in the demo below is Edit > Transform > Warp, which offers a choice of warp envelope shapes. I chose Arch.

After un-bending, it turns out there are still distortions in the tread that was traced. So next, I used Edit > Transform > Distort to align the tracing more closely with the grid that was made visible for reference. That makes it look like a better plan view.

At the end, you see that I go into Arch again because I realize it could have been unbent a little more. Instead of applying Arch a second time, it might be better to convert the layer to a Smart Object first so that if you need to adjust Arch settings later, it’s still just one adjustment, better preserving image quality. 

Draw these as paths or better yet, use Illustrator.