alice writes M1 content=v1-e9e90a90ab624fb7a0c28e900f8626b5 on node-1
  M1 id=8f3312df-2ab0-4e24-a808-28a4224128e5
settle 5s for initial replication
bob updates M1 content=v2-71f571adf3b1425bb287646e1466555c 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="v2-71f571adf3b1425bb287646e1466555c" agent_id="ai:alice"