build: add systemd autostart for the receiver

Ship systemd/screencast-receiver.service, a template the install
script substitutes (binary path, run user) and enables so the
receiver starts at boot after the network and avahi-daemon, with
restart-on-failure and a raised start limit for slow-booting boards.
SIGTERM gives the graceful shutdown that withdraws the mDNS
announcement — systemctl stop never triggers the stale-record trap
that kill -9 does. Headless consoles render via KMSDRM (the script
adds the run user to video/render/input); desktop autologin sessions
uncomment two documented Environment lines.

The install script gains SC_RECEIVER_SERVICE=0 to opt out.

Validated: systemd-analyze verify on the substituted unit (only the
expected missing /usr/local/bin path on this dev machine), and a live
lifecycle test as a user unit: the service-managed receiver announced
itself (--discover found it), systemctl stop produced a clean exit
(Result=success, no restart) and the mDNS announcement was withdrawn
immediately.

systemd/screencast-receiver.service

@@ -0,0 +1,46 @@
+# screencast-receiver.service — autostart for the receiver.
+#
+# This file is a template: scripts/install-receiver.sh substitutes
+# __SC_RECEIVER_BIN__ and __SC_RECEIVER_USER__ before installing it to
+# /etc/systemd/system/screencast-receiver.service.
+#
+# Display environments:
+#
+# 1. Headless console (SDL renders via KMSDRM): works as installed. The
+#    configured User needs access to /dev/dri and the input devices; the
+#    install script adds it to the video/render/input groups.
+#
+# 2. Graphical session (Wayland or X11 desktop with autologin): uncomment
+#    the Environment lines in [Service] and adjust them to the session
+#    owner (XDG_RUNTIME_DIR usually lives in /run/user/1000).
+
+[Unit]
+Description=Screencast receiver (H.264 over RTP/UDP)
+Wants=network-online.target
+After=network-online.target avahi-daemon.service
+# Give slow-booting boards (and desktop sessions that start late) a chance;
+# systemd's default limit would give up after 5 fast failures.
+StartLimitIntervalSec=30
+StartLimitBurst=10
+
+[Service]
+Type=simple
+User=__SC_RECEIVER_USER__
+ExecStart=__SC_RECEIVER_BIN__ --receive
+# SIGTERM (systemctl stop) shuts the receiver down gracefully, which
+# withdraws its mDNS announcement. Never SIGKILL it; the systemd default
+# KillMode escalates only after the graceful timeout.
+KillSignal=SIGTERM
+TimeoutStopSec=10
+Restart=on-failure
+RestartSec=3
+NoNewPrivileges=true
+PrivateTmp=true
+
+# Graphical session: uncomment and adjust to the session owner.
+#Environment=XDG_RUNTIME_DIR=/run/user/1000
+#Environment=WAYLAND_DISPLAY=wayland-0
+#Environment=DISPLAY=:0
+
+[Install]
+WantedBy=multi-user.target