alice writes M1 content=v1-b2b1f36b62f34e1f87e42ba5cf183cd0 on node-1
  M1 id=12d72aa7-3174-42a5-9066-efe8ac366bf5
settle 5s for initial replication
bob updates M1 content=v2-8a2dca48c5b348cf9b771807d21caed9 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="v2-8a2dca48c5b348cf9b771807d21caed9" agent_id="ai:alice"