alice writes M1 content=v1-66f77bdc736a4599a5ab015b2705d6bf on node-1
  M1 id=4ced6a03-ab86-4373-9b75-ee340862d261
settle 5s for initial replication
bob updates M1 content=v2-cb6ae03226b94a8484977eccca3d809b 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-cb6ae03226b94a8484977eccca3d809b" agent_id="ai:alice"