Address review feedback: the stored query context loop in
_collect_sortable_identifiers omitted all_columns, silently rejecting
legitimate guest sorts on table charts configured with 'show all
columns'. Add it, plus a regression test and clarifying comments on the
order-by union and the intentional stored_values asymmetry.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>