alice writes M1 content=v1-ddbadc6bd9d74417815ee677a4a09b46 on node-1
  M1 id=ce7ec6bd-cec2-44f2-87ad-3a12571d5704
settle 5s for initial replication
bob updates M1 content=v2-3f6dbbe942ba43d0a3775031aabc3849 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="v2-3f6dbbe942ba43d0a3775031aabc3849" agent_id="ai:alice"