The receiver does not need a window manager: SDL3's KMSDRM backend
renders straight to the kernel display pipeline, which is the right
setup for a small-board appliance. But a systemd system service has
no controlling terminal, and SDL's KMSDRM backend expects one for
its VT handling — so the headless path would have failed at boot.
Give the unit StandardInput=tty with TTYPath=/dev/tty1 (harmless in
desktop mode), and document the headless recipe in the unit, the
install script output, and the RUNBOOK: Raspberry Pi OS Lite with
Console Autologin, no desktop enabled (a compositor would hold the
DRM master), and consoleblank=0.
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