`link_replies` makes an annotation whose /IRT leads nowhere a comment of
its own. For a status or checkmark that meant a card in the Comments
panel (and an invisible hit area on the page, and a root in
comment_list) that the header's count left out; before threading by
reference, most of these were hidden. The realistic case is a comment
deleted from another page than its status replies.
`Annotation::is_comment` (not a reply, not a status or checkmark) is now
the one test for "listed as a comment": the panel's cards, page badges,
filter menus and count, the page's hit-testing, comment_list's roots and
the summary all use it. A Replace Text caret still shows in its
strikeout's card: it holds the replacement text, and the count already
treats the pair as one comment.
From CodeRabbit review on #4061.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>