alice writes M1 content=v1-6a18062ce47748428bc2504923080476 on node-1
  M1 id=ebf19d40-e73c-4976-a7d0-a5f38bf6ae5b
settle 5s for initial replication
bob updates M1 content=v2-9ae7f68c748643ceb07ec8b8e0d41dcc 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="v2-9ae7f68c748643ceb07ec8b8e0d41dcc" agent_id="ai:alice"