Optional vanilla-store subscription regression traces

[PerchAnton]

I built an external adapter and replay example for small, shareable regression traces. Three scenarios exercise zustand 5.0.16 createStore and subscribeWithSelector: irrelevant/no-op updates, unsubscribe, replacement and initial-state isolation. All 13 steps pass.

This checks vanilla store behavior; React, persist middleware and storage are outside its scope. Following CONTRIBUTING, no direct upstream PR is proposed.

I maintain the experimental interaction-check runner. The example uses pinned stable packages, stays outside the project and adds no runtime dependency. Feedback on whether this helps share small reproducers is welcome.