A data platform's catalog for pickers, as its builder wrote it: what the platform
holds, described the way its users choose it, where the platform's own API isn't
open to browsers or is too big to send whole. The service doesn't read it; its
JSON is between its builder and its picker. cellxgene (cmd/cellxgene-catalog) is a
CZ CELLxGENE Discover Census release's datasets: { release, openDataset, prefix,
datasets }, a view of some being the openData source openDataset with prefix,
naming their files as paths. 404 for a catalog that hasn't been written.