alice writes M1 content=v1-dd1d753430cf402f8bf09bdf9ba15b33 on node-1
  M1 id=1589856a-8777-405e-a45b-6bb8ce03ecc2
settle 5s for initial replication
bob updates M1 content=v2-a097732f4e9741a793fc6ae2a29976ea 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-a097732f4e9741a793fc6ae2a29976ea" agent_id="ai:alice"