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/gamescope-hyperhdr.env.example

@@ -0,0 +1,23 @@
+# ~/.config/gamescope-hyperhdr/env — every flag can also be set here.
+
+# HyperHDR host (required)
+GSHDR_HOST=192.168.1.50
+#GSHDR_PORT=19400
+
+# Must be lower (= more important) than the USB grabber's priority (240)
+#GSHDR_PRIORITY=150
+#GSHDR_ORIGIN=gamescope
+
+# Frame size sent to HyperHDR and CPU supersampling factor (gamescope renders SIZE x N)
+#GSHDR_SIZE=320x180
+#GSHDR_SUPERSAMPLE=1
+
+#GSHDR_MAX_FPS=30
+#GSHDR_KEEPALIVE_MS=1000
+# Clear the priority after this many seconds without new frames (0 = off)
+#GSHDR_IDLE_CLEAR_SECS=0
+
+# node.name of the gamescope PipeWire node
+#GSHDR_TARGET=gamescope
+
+#RUST_LOG=info