build(packaging): add the Arch package, systemd unit and gamescope config

The PKGBUILD builds from the surrounding source tree (makepkg's -flto is
disabled because it breaks the libspa-sys C shim). The user unit and the
env example ship the intended deployment, and gamescope-tv.conf documents the
gamescope session it is meant to run against.

contrib/systemd/gamescope-hyperhdr.service

@@ -0,0 +1,25 @@
+# User unit for gamescope-hyperhdr. The Arch package installs it into /usr/lib/systemd/user;
+# for a manual install copy it to ~/.config/systemd/user. It runs as the user of the PipeWire
+# session and waits for the gamescope node, so it can simply stay enabled. To run it only
+# together with gamescope-tv.service, add the gamescope-tv.conf drop-in.
+[Unit]
+Description=gamescope → HyperHDR ambilight feed
+After=pipewire.service wireplumber.service
+Wants=pipewire.service
+
+[Service]
+Type=simple
+# System-wide settings, overridden by the per-user file.
+EnvironmentFile=-/etc/gamescope-hyperhdr.env
+EnvironmentFile=-%h/.config/gamescope-hyperhdr/env
+# No absolute path: systemd searches /usr/local/bin and /usr/bin, which covers both the package
+# and a manual install.
+ExecStart=gamescope-hyperhdr
+Restart=on-failure
+RestartSec=2
+# Exit code 2 is a configuration error (e.g. GSHDR_HOST not set); restarting will not help.
+RestartPreventExitStatus=2
+NoNewPrivileges=yes
+
+[Install]
+WantedBy=default.target