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(Result=success, no restart) and the mDNS announcement was withdrawn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.

README.md

@@ -51,7 +51,9 @@ The script installs the dependencies, builds a receiver-only binary
 (`-Dsender=false`), runs the test suite, and installs to `/usr/local/bin`
 (override with `SC_RECEIVER_INSTALL_DIR`). It needs Raspberry Pi OS Trixie
 or newer (GCC 13+ for C++20 `<format>`), builds SDL3 from source when the
-distribution does not package it, and enables `avahi-daemon`.
+distribution does not package it, and enables `avahi-daemon`. It also
+installs and enables a systemd service so the receiver starts at boot —
+`systemctl status screencast-receiver` to check on it.
 
 Performance note: decoding is software H.264; on very small boards expect
 smooth playback for modest resolutions and reduced frame rates at high