alice writes M1 content=v1-b167e850c3e34d889a0772d3c57bf4d1 on node-1
  M1 id=86ce8ab6-b82a-41ad-8304-2025c306ccdd
settle 5s for initial replication
bob updates M1 content=v2-c0ea30f78af848b980511b3aac90f275 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-c0ea30f78af848b980511b3aac90f275" agent_id="ai:alice"