The count left out DISPLAY_VISIBLE_SQL, so model-only rows from
micro-compaction were counted while the reader painted none, and the history
wait this count exists to skip still ran. SessionDB.display_message_count now
counts the same set _display_rows_from_conn paints, through _read_one instead
of reaching into db._lock/_conn, and one _live_count_field serves all three
listing paths. The field is declared on SessionListRow, ProfileSessionPreview
and ProfileCanonicalSession (extra=forbid) and the desktop derives it from the
generated contract instead of hand-typing it twice.
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