alice writes M1 content=v1-c764b9210d5c4bc1bd9e04137b6dbc30 on node-1
  M1 id=8500e806-8a3d-4101-afba-137a8d1c41d5
settle 5s for initial replication
bob updates M1 content=v2-3f5a335ba6284219ab6d759d7dfd09be 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="v2-3f5a335ba6284219ab6d759d7dfd09be" agent_id="ai:alice"