alice starts a session on node-1
  session_start returned HTTP 200, session_id=552e71aa-4dce-44aa-91a7-f93d9dbcfb87
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=552e71aa-4dce-44aa-91a7-f93d9dbcfb87
  bob sees 2 rows tagged session_id=552e71aa-4dce-44aa-91a7-f93d9dbcfb87 (expected 2)