add handling for irresolvable conflicts

[ajtmccarty]

add the ability to mark certain diff conflicts as irresolvable. right now, this is all node-level delete conflicts. for example, if I delete a node on main and update it on branch, we will block merging until the conflict is resolved manually, either by deleting the node on branch or undoing the changes on branch

also makes some changes to DiffCombiner b/c it was not correctly handling a manual undo of a conflict in some cases

[codspeed-hq]

CodSpeed Performance Report

Merging #4939 will not alter performance

_{Comparing ajtm-11142024-irresolvable-conflicts (39530fc) with develop (f00a6ae)}

Summary

✅ 10 untouched benchmarks