Portable Buffer reproducers for editing and validation

[PerchAnton]

Would portable Buffer event traces be useful as an optional testing example?

I built an external adapter calling the real Buffer API. Three scenarios cover Unicode cursor edits, explicit undo checkpoints, and validator errors: all 21 steps and 21 selected native Buffer/Document tests pass locally.

I maintain the experimental interaction-check runner. It adds a JSON format for sharing these sequences and remains separate from application dependencies. This is buffer-level coverage; application input, key bindings and terminal behavior still need their native tests. The existing testing question #1243 concerns that broader layer.