Review round 4:
- A switch checks the page it leaves again after flushing it, so a
  deletion that lands meanwhile still tells the user; the prepared page
  transition is unwound.
- discardDeletedPage also drops work whose first save could not be
  verified, and recomputes the queue's error from what remains, so a
  dropped op's error no longer holds the canvas on the deleted page.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>