Steps open across calls

open(scope, label) returns { commit(), cancel() }, and record is built on it. A brush stroke or a placement spans pointer events, so a callback cannot hold it. Undo and redo return false while a step is open, and an open inside another step joins it.

cancel() files nothing and keeps the changes made so far. A caller that wants them gone undoes them itself, as voxel-map's lift does with unrecorded writes.

Consequences

• A caller that never commits leaves undo disabled; editors commit on pointer release and on dispose.