alice writes M1 content=v1-5ae40efcce14482796b0441824a4c231 on node-1
  M1 id=0c7b9264-8207-4d6d-98ab-c9b5fb8a41f2
settle 5s for initial replication
bob updates M1 content=v2-24e568058fb34612a6890ac26927a560 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="v2-24e568058fb34612a6890ac26927a560" agent_id="ai:alice"