alice writes M1 content=v1-22be1324180a4b33b3e55f4103876ba6 on node-1
  M1 id=1e591aaf-9258-45d7-bf66-6af8d713a2bd
settle 5s for initial replication
bob updates M1 content=v2-bc73f450881940e3b6020a215e64b48e 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-bc73f450881940e3b6020a215e64b48e" agent_id="ai:alice"