alice writes M1 content=v1-634d3fa60c9647f69e3bb9c7dab27b9f on node-1
  M1 id=b9ad0c09-1508-4ab3-b8e3-029fceb60722
settle 5s for initial replication
bob updates M1 content=v2-45ca2b21a63e4fb1a499a8a4a60fc591 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="v2-45ca2b21a63e4fb1a499a8a4a60fc591" agent_id="ai:alice"