alice writes M1 content=v1-160ba8e0f58643adb3fde78f0d42fbec on node-1
  M1 id=ad8e4f8e-f7b1-4e25-9772-f356280bba46
settle 5s for initial replication
bob updates M1 content=v2-7d61ff4dcd564646a4e5ff973a1b767e 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="v2-7d61ff4dcd564646a4e5ff973a1b767e" agent_id="ai:alice"