alice writes M1 content=v1-188090c618c34a84853e2d5bff8ebd96 on node-1
  M1 id=ceb2fca5-21f8-4125-a87a-feef4482a5cc
settle 5s for initial replication
bob updates M1 content=v2-18b733f5aef3443482d82a6ce4c2e07c 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="v2-18b733f5aef3443482d82a6ce4c2e07c" agent_id="ai:alice"