alice writes M1 content=v1-a60b4eda399146a4994c93bea741b116 on node-1
  M1 id=97595f0e-df96-480c-86b8-e26856b5b862
settle 5s for initial replication
bob updates M1 content=v2-f171ad18a1b54524b71a8848f8d7f47e 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-f171ad18a1b54524b71a8848f8d7f47e" agent_id="ai:alice"