A published image is an index over several architectures; pulled, the runtime's
store keeps the index and refuses to push one platform out of it. The builder now
reads the index and every manifest it names over the registry API, with the
anonymous bearer token the public hub hands out, moves each blob by digest into the
mesh's registry, puts the manifests and then the index under the module's repository,
and pins the index. Genesis, with no registry to copy into, keeps the pull
(novox/hq 04-ISSUES/046, ADR 0096).
