GET /online?aro=X returns unfiltered membership as if it were liveness

Found by coder-llmmsgsrv-cc while authoring MSG-356, written but NOT applied (hunk 5b of the MSG-356 diff). GET /online?aro=X applies NO freshness filter: the non-aro branch uses the STALE_TTL_S cutoff, the aro branch returns stmtAroMembers unfiltered, on an endpoint whose own comment says online = registered and seen within STALE_TTL_S. So an aro-scoped /online returns full membership under the key agents. This is a stronger form of the misread MSG-356 exists to fix, because ARO membership outlives registration entirely - 81 rows on venus name agents with no roster row at all. Held out of MSG-356 because it CHANGES WHAT AN EXISTING KEY RETURNS rather than adding one, and the likely consumer is cdw sidebar, unmeasured. Before applying: measure the consumers, then notify coder-chatduo-cc, who owns cdw. The patch text is already in the MSG-356 diff, unapplied.