alice writes M1 content=v1-ba7836f4b0ca41caae1383754cd2f91f on node-1
  M1 id=2052b64d-8fc0-4a18-b747-889beefabeaf
settle 5s for initial replication
bob updates M1 content=v2-56b43b01d8014cd1aacfe7ead9fa93b7 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="v2-56b43b01d8014cd1aacfe7ead9fa93b7" agent_id="ai:alice"