Package routes share an origin with /ui, and serve bodies whose
Content-Type comes from upstreams or from whoever pushed a hosted package.
An artifact served as text/html or image/svg+xml ran its scripts with
access to the UI.
- Send X-Content-Type-Options: nosniff and Content-Security-Policy: sandbox
on every package-route response: cached and streamed artifacts, cached
metadata, relayed upstream responses and errors
- Archive files browsed from /ui already get the same two headers
