The memory of an editor's failed flush also caught viewers whose read had
failed and recovered, blocking their exports, and was lost on a page
switch. A demoted editor's edits are already refused and shown on screen
(#284), so exporting the server's copy matches what they were told; the
case is out of scope here. The draft check only served that case: an
editor's flush already queues drafts with the canvas.

A viewer whose read failed and recovered can export again (test).

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