alice writes M1 content=v1-fd15b13a0f5c410198467dcdd077f952 on node-1
  M1 id=1de0035a-1304-4f53-b2db-14c1b8454c50
settle 5s for initial replication
bob updates M1 content=v2-caa3482b590c44fda16bd6b273a20bde 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-caa3482b590c44fda16bd6b273a20bde" agent_id="ai:alice"