alice writes M1 content=v1-a574bc9a080945debf749f6c695b340c on node-1
  M1 id=d6202ff6-2498-4576-b958-b57566aa32d0
settle 5s for initial replication
bob updates M1 content=v2-be554ca3f9b94e8a8eb31afb7846d4cb 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-be554ca3f9b94e8a8eb31afb7846d4cb" agent_id="ai:alice"