feat(packaging/arch): Arch + SteamOS install target (PKGBUILD + sysext)

Add packaging/arch: a PKGBUILD mirroring the rpm/deb artifact set (binary, udev
rule, 32MB sysctl, systemd USER units with ExecStart rewritten, headless helpers,
env templates, openapi), a pacman .install scriptlet, a systemd-sysext builder for
immutable SteamOS, and a README. Builds the working tree via PF_SRCDIR (CI/dev) or
a git tag (AUR). Arch's stock ffmpeg already ships NVENC, so deps collapse to ~10
packages with nvidia-utils/compositors as optdepends (never hard-depend on the
driver, same invariant as rpm/deb).

SteamOS delivery is a **systemd-sysext** (overlays /usr read-only from writable
/var/lib/extensions/, survives A/B OS updates, no steamos-readonly disable) —
pacman/distrobox/flatpak are all unsuitable for a host that needs uinput/uhid, the
host PipeWire socket, the GPU node, and to spawn a compositor.

KNOWN GAP, documented prominently: encode is NVENC-only (src/encode/linux.rs has no
VAAPI backend), so this works on Arch+NVIDIA (and bazzite-deck-nvidia) but an AMD
Steam Deck installs yet cannot encode until a hevc_vaapi backend is written — a code
change, not packaging.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packaging/arch/punktfunk-host.install

@@ -0,0 +1,25 @@
+# pacman install scriptlet — mirrors the RPM %post / deb postinst.
+post_install() {
+  udevadm control --reload-rules 2>/dev/null || true
+  udevadm trigger --subsystem-match=misc 2>/dev/null || true
+  # Apply the UDP socket-buffer tuning now (also auto-applied at boot by systemd-sysctl).
+  sysctl -p /usr/lib/sysctl.d/99-punktfunk-net.conf >/dev/null 2>&1 || true
+  cat <<'MSG'
+punktfunk-host installed.
+  1. Add yourself to the 'input' group for virtual gamepads:
+       sudo usermod -aG input "$USER"     # then re-login
+  2. Pick a backend config (gamescope is the no-desktop default on SteamOS/Deck):
+       mkdir -p ~/.config/punktfunk
+       cp /usr/share/punktfunk/host.env.bazzite ~/.config/punktfunk/host.env
+  3. Enable the host:
+       systemctl --user enable --now punktfunk-host
+
+NOTE: encode is NVENC-only. Install 'nvidia-utils' on an NVIDIA host. An AMD Steam Deck is NOT
+      yet supported — it needs a VAAPI (hevc_vaapi) encoder backend (see packaging/arch/README.md).
+MSG
+}
+
+post_upgrade() {
+  udevadm control --reload-rules 2>/dev/null || true
+  sysctl -p /usr/lib/sysctl.d/99-punktfunk-net.conf >/dev/null 2>&1 || true
+}