Headless EditorState and history traces as shareable JSON fixtures

[PerchAnton]

I built an external adapter and replay example for small, shareable regression traces. Three scenarios exercise the matching 0.52.0 headless/history packages: Unicode replacement, undo/redo, immutable saved EditorState and JSON round trips. All 16 steps pass.

This exercises editor APIs, not keyboard events or React components; it does not solve the jsdom input problem in #2659.

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.