One aggregation-level fact per SELECT, populated when the statement
carries a HAVING clause. Blueprint §S16. The
having text is
round-tripped from
THavingClause.toString() — dialect-faithful
verbatim text (lowercased to match the existing convention) so a
downstream BFF can echo "this query filters groups by …" without
re-parsing.
Wire-format contract: empty list when the statement has no
HAVING; one entry per statement that does. Currently single-field;
future S16 work may add groupBy / having.atoms[] —
the typed shape leaves room without breaking consumers.