alice writes M1 content=v1-fa34090070714171b4f78a6f45d61c89 on node-1
  M1 id=34ea9f4d-0834-48c4-9877-c79a34437f5c
settle 5s for initial replication
bob updates M1 content=v2-47f12918451d4827974a76b9f45762e0 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="v2-47f12918451d4827974a76b9f45762e0" agent_id="ai:alice"