packaging(nix): add a Nix flake for reproducible from-source builds

Add a flake that builds all four binaries (fips, fipsctl, fips-gateway,
fipstop) on Linux and macOS, pinning the exact toolchain from
rust-toolchain.toml (1.94.1 + rustfmt/clippy) via fenix so Nix builds
match CI and the AUR/Debian packaging.

Wire up the build-time native deps the source tree needs: pkg-config and
bindgenHook (libclang) for the rustables/libdbus-sys bindgen step, and
dbus for bluer's BLE support. autoPatchelfHook rewrites the binary RPATHs
so the daemon resolves libdbus-1.so.3 and libgcc_s.so.1 from the Nix store
at runtime — without it `fips` fails to load on NixOS, which has no global
/usr/lib.

Outputs: packages.{default,fips}, apps for each binary, checks.fips, and a
devShell with the pinned toolchain plus cargo-edit. Tests are skipped in
the package build since they exercise TUN devices, raw sockets, and mDNS
that aren't available in the sandbox, mirroring the AUR/Debian packaging.

Verified end-to-end in a pure Nix store (nixos/nix container): nix build,
nix flake check, and running all four binaries succeed against the
committed flake.lock.

Documented across the install and developer docs: a Nix / NixOS section in
packaging/README.md, the from-source guide in docs/getting-started.md
(noting the flake produces binaries only, with NixOS system integration
through the system configuration rather than the installer), the CHANGELOG,
README, CONTRIBUTING, and the v0.4.0 release notes.

Co-authored-by: Johnathan Corgan <johnathan@corganlabs.com>

docs/getting-started.md

@@ -88,6 +88,23 @@ See [packaging/README.md](../packaging/README.md) for per-format
 build details, cross-target options, and the full `make` target
 list.
 
+### With Nix (flake)
+
+On Nix/NixOS, a [flake](../flake.nix) at the project root builds the
+binaries from source with the pinned toolchain and no manual
+prerequisite install:
+
+```sh
+nix build .#fips          # all four binaries, into ./result/bin
+nix develop               # dev shell with the toolchain + build deps
+```
+
+This path produces binaries only — it does not run the installer, so
+there are no systemd units, no `fips` group, and no default `fips.yaml`.
+On NixOS, wire the daemon in through your system configuration using the
+flake's `packages.<system>.fips` output instead. See the Nix / NixOS
+section of [packaging/README.md](../packaging/README.md).
+
 ## What's installed and running
 
 Here's what the installer leaves on your machine, what's

docs/releases/release-notes-v0.4.0.md

@@ -257,6 +257,9 @@ Operator-actionable items moving from v0.3.0 to v0.4.0:
 - **From source**: `cargo build --release` from a checkout of the v0.4.0
   tag (Rust 1.94.1 per `rust-toolchain.toml`; `libclang-dev` is a
   required Linux build prerequisite).
+- **Nix / NixOS**: `nix build .#fips` from a checkout of the v0.4.0 tag
+  builds the binaries from source with the pinned toolchain and no manual
+  prerequisites (see the Nix section of `packaging/README.md`).
 
 The full per-commit changelog lives in
 [`CHANGELOG.md`](../../CHANGELOG.md). Issues and discussion at