alice writes M1 content=v1-d2168402cdab4833aecdbe4d2aa4a2ae on node-1
  M1 id=9e154905-49e7-4c2a-b91a-1c9154d2cf44
settle 5s for initial replication
bob updates M1 content=v2-3b1338bd598a4361841ab5f5cee9d399 on node-2 via PUT
  PUT returned HTTP 200
settle 8s for update fanout
charlie reads M1 on node-3 and checks content + provenance
  charlie sees content="v2-3b1338bd598a4361841ab5f5cee9d399" agent_id="ai:alice"