alice writes M1 content=v1-8286f157cb7a47f88c36b24c8149be61 on node-1
  M1 id=bc6d68e7-d8d1-4a0f-8e04-7b258890d541
settle 5s for initial replication
bob updates M1 content=v2-fb0475cf6d6b43c18d7afd8504b2b02d 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-fb0475cf6d6b43c18d7afd8504b2b02d" agent_id="ai:alice"