release: nix flake with tag-pinned hashes, reproducible tarballs (milestone 40)

flake.nix fetches the release tarballs and carries their SRI hashes in a generated block. The cut tool builds the release locally with the toolchain gates.yml pins, in a normalized nine-variable environment, writes the hashes into flake.nix, and commits it with build.zig.zon as the single bump commit. The package job verifies the pins on the bump commit and the publish job verifies them again on the tag, before anything is uploaded.

The tarballs are written by dist_stage (std.tar.Writer, flate gzip) instead of the runner's tar and gzip, and -ffile-prefix-map keeps checkout paths out of the C objects; two checkouts at different absolute paths produce byte-identical archives. nxdns version, /api/version and the admin footer report the version only: the bump commit cannot know its own sha.

docs/README.md

@@ -22,6 +22,7 @@ Steps for a goal you already have. They assume you know what nxdns is.
 - [how-to/verify-a-release.md](how-to/verify-a-release.md) — check the signature and the checksums before you run anything, and what they prove.
 - [how-to/install-with-systemd.md](how-to/install-with-systemd.md) — a real install as a system service, including the Raspberry Pi 5 aarch64 binary.
 - [how-to/install-with-docker.md](how-to/install-with-docker.md) — the published container image and the compose file.
+- [how-to/install-with-nix.md](how-to/install-with-nix.md) — the flake input pinned to a release tag, and Renovate for tag bumps.
 - [how-to/upgrade.md](how-to/upgrade.md) — move to a new release without losing state.
 - [how-to/troubleshoot.md](how-to/troubleshoot.md) — what to do when it does not answer, does not block, or will not start.
 - [how-to/enable-doh-and-dot.md](how-to/enable-doh-and-dot.md) — serve encrypted DNS with certificates.