alice writes M1 content=v1-e496d2b7ac99484ca7d6cc830fd4536c on node-1
  M1 id=a33d5951-da88-43fc-8d08-a441ce2efafb
settle 5s for initial replication
bob updates M1 content=v2-b5329738f8b54edc86b534cf7a8d98ac 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-b5329738f8b54edc86b534cf7a8d98ac" agent_id="ai:alice"