alice writes M1 content=v1-3b51deaee5f344acbc7b58a60c5b7bb6 on node-1
  M1 id=65f8a24e-dee3-46d1-96fa-bc1e2e6a1785
settle 5s for initial replication
bob updates M1 content=v2-9d526dfc6eaf400a88e8d751c3b80600 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="v2-9d526dfc6eaf400a88e8d751c3b80600" agent_id="ai:alice"