alice starts a session on node-1
  session_start returned HTTP 200, session_id=b23a6d0d-d009-4dcd-ab15-573ee1aecadf
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=b23a6d0d-d009-4dcd-ab15-573ee1aecadf
  bob sees 2 rows tagged session_id=b23a6d0d-d009-4dcd-ab15-573ee1aecadf (expected 2)