alice writes M1 content=v1-7864325464ae400ba4b3305697359fdf on node-1
  M1 id=b02c2075-0b3d-4074-840c-38d0bb8c1df9
settle 5s for initial replication
bob updates M1 content=v2-25f7e57a7908468fb89f2fa094018f19 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="v2-25f7e57a7908468fb89f2fa094018f19" agent_id="ai:alice"