alice writes M1 content=d-b2c2f1980d5044e2843a7a3221b8c506 on node-1
  created memory id=5d366e32-b8fe-4f93-b981-4c8e9859d42b
settle 8s for pre-delete fanout
pre-delete: verifying M1 is visible on all peers
  pre-delete node-2 sees 1
  pre-delete node-3 sees 1
  pre-delete node-4 sees 1
alice deletes M1 on node-1
  DELETE returned HTTP 200
settle 15s for tombstone propagation
post-delete: verifying M1 is GONE from all peers
  post-delete node-2 sees 0 (expected 0)
  post-delete node-3 sees 0 (expected 0)
  post-delete node-4 sees 0 (expected 0)