packaging: build recipes for arch, gentoo, nix, void, fedora

Five distro recipes in packaging/, all following the same recipe:
unpack the Gitea release archive (which extracts to fastwc/), run
autoreconf -fi (the generated configure/Makefile.in are not committed),
configure, make, make install - the binary needs only glibc.

Each recipe notes the release prerequisite: the archive URL requires a
v0.1.0 tag on the remote, and Arch/Nix/Void carry an integrity
placeholder to fill in once the tag exists. The PKGBUILD was validated
with makepkg --printsrcinfo, and the full build recipe was run
end-to-end from a Gitea-layout tarball (autoreconf, configure, make,
make install DESTDIR) producing a working /usr/bin/fastwc.

packaging/README.md

@@ -0,0 +1,49 @@
+# Packaging
+
+Build recipes for the distributions that matter. Each one follows the
+same recipe: unpack the release archive, regenerate the autotools
+files (`autoreconf -fi` — the generated `configure`/`Makefile.in` are
+not committed), `configure`, `make`, `make install`. The binary is a
+single static-ish ELF that only needs glibc.
+
+| Distro | File | Tooling |
+|--------|------|---------|
+| Arch | `arch/PKGBUILD` | `makepkg` |
+| Gentoo | `gentoo/app-misc/fastwc/fastwc-0.1.0.ebuild` | `ebuild` / `emerge` (drop into a local repo or `app-portage/eselect-repository`) |
+| NixOS | `nix/default.nix` | `nix build -f default.nix` (or a `callPackage` from a nixpkgs overlay) |
+| Void | `void/template` | `xbps-src pkg fastwc` (in a `void-packages` checkout: `cp -r ../fastwc/packaging/void srcpkgs/fastwc`) |
+| Fedora | `fedora/fastwc.spec` | `rpmbuild -ba fastwc.spec` (or `fedpkg`/`mock`) |
+
+## Before the first build
+
+Each recipe points at the Gitea release archive
+`https://git.spectoria.dev/huntedbytheirs/fastwc/archive/v0.1.0.tar.gz`,
+which does not exist until a `v0.1.0` tag is pushed:
+
+```sh
+git tag -a v0.1.0 -m "fastwc 0.1.0"
+git push origin v0.1.0
+```
+
+Then fill in the integrity placeholders each distro demands:
+
+- **Arch**: replace `sha256sums=('SKIP')` with the hash `makepkg` reports on the first attempt.
+- **NixOS**: `lib.fakeHash` makes the first build fail with the real hash — paste it in.
+- **Void**: replace `checksum=SKIP` with the sha256 `xbps-src` wants (it will refuse to build until then).
+- **Gentoo / Fedora**: hashes live outside the recipe (the `Manifest` file / source RPM metadata), so nothing to fill in here.
+
+## Notes
+
+- **Gitea archive layout**: these archives extract to a top-level
+  directory named after the repo (`fastwc/`, no version suffix), which
+  is why the recipes `cd fastwc`, set `S="${WORKDIR}/fastwc"`, or use
+  `%autosetup -n fastwc`. If a future host renames the layout, adjust
+  those three spots.
+- **Architecture**: the SIMD kernels dispatch AVX-512/AVX-2/SSE2 at
+  runtime with a scalar fallback, so the code builds anywhere — the
+  recipes currently declare `x86_64` because that is what the benchmark
+  contract is validated on. Loosen if you build elsewhere.
+- **`-pthread`** is in `AM_CFLAGS`; glibc >= 2.34 provides pthreads in
+  libc, so there is no separate runtime dependency.
+- The `fastwc` binary installs to `$(bindir)/fastwc` and does not
+  shadow `wc` — nothing conflicts with coreutils.