A peer edit refuses the whole step

When a peer writes a value a step would write back, the step is refused and undo passes over it, emitting skipped. Nothing of it is undone. VoxelHistory used to undo the cells nobody touched and silently keep the others.

A refusal is visible: state(scope).refused lists it, and editors log the skipped step. A partial undo leaves a state nobody made, and the user cannot tell which part came back.

The same rule covers writes the history never saw. Before replaying, undo and redo check that each part's guard still holds its capture, and refuse the step as changed otherwise. A voxel layer moved by this client reports no cell, so a step on the moved cells is refused at undo instead of writing them at their old world positions.

Considered Options

• Skip the changed values, undo the rest (the former voxel behaviour). Silent, and a stroke half undone is still a step the user did not make.
• Report layer moves as writes. A local write recaptures the guard, which would make the stale step undoable again, at the wrong positions.

Consequences

• An undo can do nothing but skip, and say why.
• A document's guard only needs to cover what its undo writes; anything else that changes those values is caught at undo.