build: make the receiver service headless-console ready

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.

docs/RUNBOOK.md

@@ -90,6 +90,22 @@ Verified behavior: `systemctl stop` results in a clean exit (success,
 no restart) and the mDNS announcement is withdrawn — senders stop finding
 the receiver immediately.
 
+## Headless receiver (no window manager)
+
+The receiver does not need X11, Wayland, or any window manager: SDL3's
+KMSDRM backend renders straight to the kernel display pipeline. This is
+the recommended setup for small boards (Raspberry Pi OS **Lite**):
+
+1. `sudo raspi-config` → System Options → Boot → **Console Autologin**.
+2. No desktop may be enabled — a running compositor holds the DRM master
+   and the receiver cannot take it (the two modes are mutually exclusive).
+3. Disable console blanking: append `consoleblank=0` to
+   `/boot/firmware/cmdline.txt` and reboot.
+
+A window manager only comes with the desktop-session alternative, where
+the receiver runs inside it (uncomment the `Environment=` lines in the
+service file as described above).
+
 ## Sender / receiver loopback (Phase 5, manual)
 
 Two terminals on the same desktop session:

scripts/install-receiver.sh

@@ -138,13 +138,17 @@ The receiver is installed and starts automatically at boot.
     sudo systemctl disable --now screencast-receiver   # turn autostart off
 
 Notes for small boards (e.g. Pi Zero 2 W):
+  - No window manager is required: the receiver renders directly to the
+    kernel display pipeline (SDL KMSDRM). Use Raspberry Pi OS Lite with
+    Console Autologin (raspi-config) and no desktop enabled — a running
+    compositor would own the display and the receiver could not start.
+  - Disable console blanking so the picture never goes dark: add
+    consoleblank=0 to /boot/firmware/cmdline.txt and reboot.
   - Decoding is software H.264; expect smooth playback for small streams
     and a lower frame rate at high resolutions.
   - Stop the receiver via systemctl (never kill -9; stale mDNS records
     would break discovery for ~75 minutes).
-  - The service runs on the headless console via KMSDRM. On a desktop
-    session instead, edit /etc/systemd/system/screencast-receiver.service:
-    uncomment the Environment lines and point XDG_RUNTIME_DIR at the
-    session owner's /run/user/<uid> (see the comments in that file).
+  - On a desktop session instead, edit the installed service file and
+    uncomment the Environment lines for XDG_RUNTIME_DIR/WAYLAND_DISPLAY.
   - avahi-daemon must stay enabled; it is what senders discover.
 EOF

systemd/screencast-receiver.service

@@ -27,6 +27,11 @@ StartLimitBurst=10
 Type=simple
 User=__SC_RECEIVER_USER__
 ExecStart=__SC_RECEIVER_BIN__ --receive
+# SDL's KMSDRM backend (headless console) expects a controlling terminal
+# for its VT handling; system services have none by default. Harmless in
+# desktop mode, where the process never touches the tty.
+StandardInput=tty
+TTYPath=/dev/tty1
 # 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.
@@ -44,3 +49,11 @@ PrivateTmp=true
 
 [Install]
 WantedBy=multi-user.target
+
+# Headless console recipe (no window manager needed — SDL renders directly
+# to the kernel via KMSDRM):
+#   * Raspberry Pi OS Lite, no desktop enabled (a running compositor would
+#     hold the DRM master and the receiver could not take it)
+#   * raspi-config → System Options → Boot → Console Autologin
+#   * disable console blanking: add consoleblank=0 to
+#     /boot/firmware/cmdline.txt