alice writes M1 content=v1-08881a8646e944d4a417702b4aaec786 on node-1
  M1 id=b2d563ed-dace-466b-bdb4-b650bcef246a
settle 5s for initial replication
bob updates M1 content=v2-b98f81fff4864e0380e993249cad2c6d 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-b98f81fff4864e0380e993249cad2c6d" agent_id="ai:alice"