The fork checkpoint check refused a fork when any projected row spanned the
selected reply. A plugin card re-appended with the same id keeps its first
position and stretches to its latest update, so it spanned every reply in
between, although the fork never copies plugin cards. The check now looks only
at the row kinds the fork copies, taken from the same constant as the copy.
Closes#8745
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>