npm i && npm start
kvPersist writes disk by filename matching 1:1 with the entry key, keys with / included will write out a directory structure.
This means creating key like foo/bar creates a file .mf/kv/TEST_NAMESPACE/foo/bar, then also creating a key like foo will find .mf/kv/TEST_NAMESPACE/foo to already be a directory, and Node.js throws EISDIR: illegal operation on a directory.