alice writes M1 content=v1-527d6b2b30cf47329fb80945c937df46 on node-1
  M1 id=a1d61ad5-e91a-4e52-9aac-c73bec4329f5
settle 5s for initial replication
bob updates M1 content=v2-b8ad0939b533453c8040cebc5c783496 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-b8ad0939b533453c8040cebc5c783496" agent_id="ai:alice"