I hope that I am reinventing the wheel by writing this post. And that some game theorist has already written something much better on the same subject. (If not, then they should have done.) But I need a game theorist to tell me about it and explain it to me, in simple language (if possible).
[The macro punchline comes at the end of this post.]
This is my crude attempt to define "fragility/robustness" of Nash equilibria: