alice writes M1 content=v1-61424dd3db024db4bb40980a763caac7 on node-1
  M1 id=a099c1ae-9ee6-4865-afdd-b6a03c274df7
settle 5s for initial replication
bob updates M1 content=v2-73b9c66d40f6436f9627da291f69438e 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="v2-73b9c66d40f6436f9627da291f69438e" agent_id="ai:alice"