Thanks to all who read and/or replied. No matter that I have close to 50 chickens, with each loss goes a bit of me. Every one is special. Only BYC folks truly appreciate that.
I'm almost sure that her death was due to low calcium. As she seized, she stiffened & was paralyzed, a symptom of calcium tetany. Perhaps she had a genetic problem with metabolizing calcium. I don't know. If I'd realized sooner that she was egg bound, I could have given her a solution of calcium in water, but I didn't. (Crushed Tums? Does anyone know how much per pound/KG of chicken?)
Though I see the occasional soft shelled egg, it happens very seldom. They spend a good part of their day in their covered day pen as well as in their secure runs, and at least an hour a day free ranging (under supervision), so they should have no shortage of Vitamin D.
It's been a very, very stressful day. Think I need to say a final goodbye to my dear Gracie & go cry for a while.