alice writes M1 content=v1-d121f7317720415ea2f240d64a4ef40c on node-1
  M1 id=df1dfe9a-57bb-4ca7-a6d5-35de52afd978
settle 5s for initial replication
bob updates M1 content=v2-03c49987b8ea4153b9376482e8562cd9 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="v2-03c49987b8ea4153b9376482e8562cd9" agent_id="ai:alice"