alice writes M1 content=v1-afdb1b0e2bf343709767ce1a366ea53d on node-1
  M1 id=e9d6dc3e-339c-4bcb-9870-6e633d470a3a
settle 5s for initial replication
bob updates M1 content=v2-f12b79a55aef4066a6aec7beb1ed73ff 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-f12b79a55aef4066a6aec7beb1ed73ff" agent_id="ai:alice"