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>
