Codex caught: `apply_payload` only swapped pages + active page +
cleared selection — it left `history.past` / `history.future`
holding snapshots of the OLD doc, plus every UI slot that may
carry a `NodeId` from the old tree (pen_in_progress,
text_editing, layer_rename, color_picker target, property_focus,
agent_settings_drag, layer_context_menu). After Open the user
could Cmd+Z back into the previous doc, or a stale `pen_in_progress
= Some(NodeId)` from before the load would point at a non-existent
row on the next press.
Wipe history both directions and clear every NodeId-carrying UI
slot + drafts + open dropdowns so the loaded doc starts on a
clean slate.
3f77778c9e