alice writes M1 content=v1-b6dc676ee3d24bd693c077069e0fea28 on node-1
  M1 id=34329763-605e-492b-922a-aa9ef701b0c5
settle 5s for initial replication
bob updates M1 content=v2-48e121a9dbf74225878caafc7bb4b1de 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="v2-48e121a9dbf74225878caafc7bb4b1de" agent_id="ai:alice"