alice writes M1 content=v1-a7ffab2a2c6149beac18b00973a29362 on node-1
  M1 id=4c095567-ee64-4c93-8c93-a2bbdfc352f3
settle 5s for initial replication
bob updates M1 content=v2-7daa70f6917e4e208c53d52865d27769 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="v2-7daa70f6917e4e208c53d52865d27769" agent_id="ai:alice"