alice writes M1 content=v1-118429b233f4497c8336227997980d9a on node-1
  M1 id=be15803a-5b5f-4dec-bca2-d54c2e9cac9a
settle 5s for initial replication
bob updates M1 content=v2-336c29c14aef4cada37530026f495424 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="v2-336c29c14aef4cada37530026f495424" agent_id="ai:alice"