docs(changelog): record the two npm packument changes

Both entries were missing from `## [Unreleased]`: the rebuild fix (#956) and
the abbreviated packument (#957), with the numbers measured on the polygon.

Co-authored-by: ivaseeq <126263471+ivaseeq@users.noreply.github.com>

Author: devitway

CHANGELOG.md

@@ -4,6 +4,10 @@
 ### Added
 - **Hash pins on S3/GCS via object metadata** — the SHA-256 integrity pin is no longer a local-filesystem-only feature. On object-store backends it is written as the user-defined `sha256` object metadata, atomically with the object, and read back on GET/HEAD, so buffered reads verify at rest and raw files get `ETag`, `If-None-Match` (304) and `If-Match` conditional overwrite on every backend. Pins are now a backend concern: the local backend keeps its NDJSON sidecar (same path and format, no migration), the object-store backend keeps object metadata, and the storage wrapper only validates keys and runs the fail-closed verify gate. Objects written before the upgrade carry no metadata and stay open-world until they are rewritten; `nora re-pin` rewrites the object on an object store, since object metadata cannot be changed in place.
 - **Raw upload integrity via `Repr-Digest` (RFC 9530)** — a raw `PUT` may declare `Repr-Digest: sha-256=:BASE64:`; NORA verifies the received body against it before committing, so a corrupted or truncated upload is rejected with `400` instead of being pinned. The pin itself is always the server-computed hash; the header only gates the commit. A `Repr-Digest` without a sha-256 entry is rejected rather than silently skipped.
+- **npm serves the abbreviated packument to installers** — `npm install` asks for `application/vnd.npm.install-v1+json`, and NORA ignored it and returned the full document to every client. The packument path now projects to npm's abbreviated shape, keeping the per-version fields an installer actually resolves on (`dependencies`, `os`, `cpu`, `engines`, `peerDependenciesMeta`, `dist`, `deprecated`) and dropping readme, maintainers, repository, per-version `description`, `scripts` and `gitHead`. Measured against a live upstream with every version preserved: lodash 247 652 → 71 989 B (−70.9%), express 804 975 → 344 703 B (−57.2%). The short form is derived locally rather than requested upstream, so exactly one canonical object stays cached per package and a short document can never displace the full one; `Vary: Accept` goes with it, because metadata is `Cache-Control: public` and the body now varies by a request header. An unparsable body is served unchanged rather than turned into an error (#957).
+
+### Fixed
+- **npm rebuilds a missing packument instead of answering 404** — a hosted package whose derived `metadata.json` was absent returned `404` while every published version was still sitting in storage. The reassembly already existed (`regenerate_packument`, which lists `versions/`, `dist-tags/` and `pkg.json`) but only the publish path reached it, so a read fell through to the upstream proxy and 404'd for a package that exists only in this registry. The read path now rebuilds when `versions/` is non-empty, serves the result and re-materializes the packument so the cost is paid once — under the same `publish_lock` as publish, so a fleet stampeding one package rebuilds it once rather than once per request, and before the namespace guard, because serving locally-owned bytes is always allowed while that guard exists to stop the upstream fetch. A name with nothing behind it still returns 404. New `nora_packument_rebuilt_total{registry}`: a non-zero rate means storage was written or restored outside NORA (#956).
 
 ## [1.2.0] - 2026-08-23