Mystery solved. It was Scruffy that was laying the soft-shelled eggs.
Not who I expected at all.
Her eggs have always had beautiful, smooth, hard-as-rock shells. They were tough eggs to crack! Perhaps she depleted her calcium or maybe her egg shell gland wasn’t working properly. Whatever the case, the sardine-yogurt-eggshell-oatmeal mash did the trick.
It took five days, but her eggs are back to normal, and praise the chicken gods she didn’t get egg-bound at all through this ordeal.
I’ll cut back on the mash, maybe giving it two or three times a week now. I think this little chicken will always need some calcium/protein hand fed to her. She doesn’t seem to want to eat any oyster shells on her own. (Ginger, on the other hand, can’t get enough of the oyster shells, but she has yet to start laying again – go figure!)