Method can_undo

Whether there is a retired commit at `head` that can be undone in the silent zone (a commit after `last_broadcast_rev`). `head == 0` is the empty history; published commits aren't silently undoable (that needs a forward reverse-delta op, deferred until transport lands). The earliest interaction (the document's loaded/created base) is *not* undoable: undoing it would rewind into the pre-base state

(&self)