alice writes M1 content=d-6b55f4256de24b0f81c70e5734f57ce8 on node-1
  created memory id=fde5076c-e7af-4bb6-978d-017dda093557
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)