Packuments list each version's provenance at dist.attestations.url on
registry.npmjs.org. npm takes only the path of that URL and resolves it
against the configured registry, so behind the proxy `npm audit
signatures` asked for /-/npm/v1/attestations/... at the proxy's root,
outside /npm, and could not verify attestations.
The rewrite now points dist.attestations.url at
/npm/-/npm/v1/attestations/{name}@{version}, and the handler serves that
path. It reads the version's attestations URL from the packument and
fetches it from the package's registry, the route's for routed packages;
a URL on another host is not followed and the registry's standard path
is asked instead. Attestations never change once published, so they are
cached as immutable metadata.
Versions blocked by the denylist get 403 and versions held back by
cooldown 404, as their tarballs do. Pushed versions have no upstream
attestations and get 404.
