Silent-zone undo of one *interaction*: revert deltas walking `head` back along first-parents until it reaches the previous interaction boundary (a delta marked `interaction_end`) or the empty root. One interaction spans several deltas (one `commit_from_runtime` batch), so undo reverts the whole run, not a single delta — matching the legacy per-interaction undo granularity. The undone interaction's
(&mut self)
source not stored for this graph (policy: none)