Portable Slate core/history regression traces

[PerchAnton]

I built an external adapter and replay example for small, shareable regression traces. Three scenarios exercise transforms and slate-history directly: Unicode selection replacement, empty deletion and redo invalidation. All 17 steps pass.

This is core-model coverage. As discussed in #5952, browser input and IME still need Playwright; this does not emulate them.

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.