Default the x11 headless desktop to GNOME (Openbox still opt-in)

Bare Openbox rendered only a blank slate root — usable but not the
desktop wanted. Make the X11/NVENC capture path render a full GNOME
session by default, with Openbox available via HEADLESS_DESKTOP=openbox
for minimal/low-power hosts.

- files/headless-desktop-gnome.service: full Ubuntu GNOME session forced
  onto the X11 path (XDG_SESSION_TYPE=x11, no dbus-run-session so it
  shares the systemd user bus). Renamed the Openbox unit to
  headless-desktop-openbox.service.
- lib/headless.sh: HEADLESS_DESKTOP (default gnome) selects the unit
  template + the packages to install (gnome-session/gnome-shell vs
  openbox/xsetroot).
- install.sh: step message + usage document HEADLESS_DESKTOP.
- status.sh: the :0 desktop check now reports which desktop is running
  (reads _NET_WM_NAME off the supporting-wm-check window, e.g.
  "GNOME Shell").
- docs: TROUBLESHOOTING §13 + FOLLOWUPS P3 updated for the GNOME default
  and the openbox toggle.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

files/headless-desktop-openbox.service

@@ -0,0 +1,27 @@
+[Unit]
+Description=Openbox session on the headless Xorg (gives Sunshine something to capture)
+# Without a window manager/desktop running on :0, Sunshine's x11 capture grabs
+# an empty black X root window — pairing, NVENC, and input all work but the
+# client sees only black. This unit renders a session onto :0 so the stream
+# shows an actual desktop. Only relevant to the X11/NVENC capture backend
+# (capture = x11); the wlr backend's compositor renders for itself.
+Requires=xorg-headless.service
+After=xorg-headless.service
+PartOf=graphical-session.target
+
+[Service]
+Type=simple
+Environment=DISPLAY=:0
+# Wait for the X server to accept connections before launching the WM.
+ExecStartPre=/bin/sh -c 'for i in $(seq 1 20); do xset -display :0 -q >/dev/null 2>&1 && exit 0; sleep 0.5; done; exit 1'
+ExecStart=/usr/bin/openbox-session
+# Paint a solid root so a connecting client sees an obvious (non-black) desktop.
+# Leading '-' = best-effort; a missing xsetroot must not fail the unit.
+ExecStartPost=-/usr/bin/xsetroot -display :0 -solid "#2e3440"
+Restart=on-failure
+RestartSec=2s
+
+[Install]
+# Lingering user manager reaches default.target without a graphical login,
+# matching the headless-boot drop-in pattern used for Sunshine.
+WantedBy=default.target