alice writes M1 content=v1-7e42d3e9586740a5b88794ecffab1bbb on node-1
  M1 id=fbab7937-feac-40bf-890d-52f32898f051
settle 5s for initial replication
bob updates M1 content=v2-7a14140cfa974ae1a3e44f406d159e66 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="v2-7a14140cfa974ae1a3e44f406d159e66" agent_id="ai:alice"