alice writes M1 content=v1-a6a9020488e04204b855a265100fe818 on node-1
  M1 id=e6203db8-40ce-44bb-8f93-77240d8abd1c
settle 5s for initial replication
bob updates M1 content=v2-3b88e13becd74cc3b847d8dd9248331c 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="v2-3b88e13becd74cc3b847d8dd9248331c" agent_id="ai:alice"