alice writes M1 content=v1-ef7c2b3b9c034875ad65065634fa170b on node-1
  M1 id=531d93cc-6591-4185-b136-fd3510b6f5a0
settle 5s for initial replication
bob updates M1 content=v2-dfdb6fd23a0b4f02af910b99e174ecfb 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-dfdb6fd23a0b4f02af910b99e174ecfb" agent_id="ai:alice"