alice writes M1 content=v1-262ab6d993e04de5b25ef4c249204261 on node-1
  M1 id=3e7495f5-b387-4338-a4d7-4680446036d2
settle 5s for initial replication
bob updates M1 content=v2-f8d1bc827ca84952a42fb757f847f884 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-f8d1bc827ca84952a42fb757f847f884" agent_id="ai:alice"