alice writes M1 content=v1-885564fcd7c6472e961e9dd813b52795 on node-1
  M1 id=16eac81e-65ca-40e4-b17b-08bde8497866
settle 5s for initial replication
bob updates M1 content=v2-5b73fdd6385d4984829c9691f30c78f9 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="v2-5b73fdd6385d4984829c9691f30c78f9" agent_id="ai:alice"