Finalise a data-plane request: disable redirect following (redirectCount = 0) on every request, then apply the ecosystem's injected credential attach.

This is the single request-finalisation point for the whole data plane: every adapter's request builder funnels through it, so pinning redirectCount = 0 here makes one invariant universal: Écluse never follows an upstream redirect, on the credentialed and the anonymous plane alike. The credential attach is injected (a Request -> Request function, an ecosystem's own scheme: npm's Bearer, another's Basic) rather than fixed here, so no attach can reach the wire around the pin.

Two dangers it forecloses, one per plane:

* Credential leakage (credentialed plane). http-client's default (redirectCount = 10) re-sends the Authorization header to the redirect's Location, and its shouldStripHeaderOnRedirect does not strip it cross-host, so a hostile or misconfigured upstream could 302 a forwarded/minted credential to an attacker-chosen host. That is especially dangerous on the trusted private manager, where a redirect could exfiltrate the credential to an attacker-chosen target; pinning redirectCount = 0 removes the hop entirely rather than relying on the per-hop egress controls.

* SSRF via redirect (anonymous plane). The host allowlist is enforced when the URL is built, not per redirect hop, so following a 302 would let an allowlisted upstream steer an anonymous fetch to any host: an internal/cloud-metadata address or any off-allowlist host: re-gated by nothing. Not following the redirect removes the hop there is to gate.

The accepted consequence, symmetric across both planes: a read no longer follows an upstream's CDN 302: it returns the 3xx to the serve path rather than chasing it. That is the safer posture, and the proxy already honours the packument's dist.tarball location explicitly, gated by the egress policy, rather than relying on redirects. Redirect-following for a nonstandard upstream (a presigned/redirecting object store) is an explicit, per-upstream opt-in, never the default.

The conditional-GET validators to relay on a metadata fetch. Replaying an upstream's ETag as If-None-Match (or its Last-Modified as If-Modified-Since) lets the upstream answer 304 Not Modified with no body: the cheap freshness check the proxy uses on a cache revalidation. Both are forwarded only when present.

No conditional-GET validators: an unconditional fetch.

Build the artifact GET request addressing a tarball at its __authoritative upstream location__: the absolute url a projection preserved from the upstream's dist.tarball, rather than reconstructing it from a (base, package, file) coordinate. The ecosystem's credential attach is injected and the request is finalised through finaliseRequest, so the redirect pin always applies.

The artifact location is server-chosen data, not a derivable fact: a registry may serve a version's tarball from a different host or a path a naming convention cannot rebuild. Honouring the preserved location is what lets Écluse front those registries; the URL it fetches is the same one the served metadata's integrity digest is paired with, so the bytes still verify.

The request is marked non-decompressing (decompress returns False): a tarball is opaque binary that must reach the client byte-for-byte, so it is never gunzipped in flight and its integrity digest stays valid. Fails with a UrlFormationError only when the url cannot be parsed into a request.