Add reproducible headless desktop for the X11/NVENC capture path

A Moonlight client connecting to the x11-backend host got a black screen
even though pairing, NVENC, and input injection all worked: the headless
Xorg on :0 had no window manager rendering on it, so capture=x11 grabbed
an empty black root window. (The wlr/kms backends don't hit this — their
capture source renders for itself.)

This was a hand-built path with nothing in the repo to reproduce the
desktop piece. Now:

- files/headless-desktop.service: Openbox session on :0, bound to
  xorg-headless.service, enabled via default.target for lingering boots,
  with a best-effort xsetroot so the desktop is visibly non-black.
- lib/headless.sh: capture_backend_is_x11 + install_headless_desktop
  (idempotent; pulls openbox/xsetroot via the distro dispatch).
- install.sh: installs the desktop unit when capture=x11 is detected.
- status.sh: x11 branch now FAILs if no window manager is on :0 instead
  of only checking the X server answers — the gap that hid this failure.
- docs: TROUBLESHOOTING §13 black-screen lesson; FOLLOWUPS P3 updated.

Part of the P3 x11-backend work; --backend flag, config.sh x11 variant,
and xorg-headless templates remain outstanding.

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

docs/TROUBLESHOOTING.md

@@ -478,6 +478,29 @@ systemctl --user show $UNIT -p Environment -p Requires -p After
 # should show DISPLAY=:0 + XDG_SESSION_TYPE=x11, xorg-headless.service, no wayland/sway
 ```
 
+**Gotcha: black screen in Moonlight even though everything "works".** Pairing
+succeeds, NVENC loads, mouse/keyboard input reaches the host — but the client
+sees only black. Cause: the headless Xorg on `:0` is up, but **nothing is
+rendering on it**. Unlike the wlr path (where the compositor *is* the desktop),
+a bare Xorg server draws nothing on its own, so `capture = x11` grabs an empty
+black root window. Confirm with:
+
+```bash
+DISPLAY=:0 xlsclients                              # only 'sunshine' = no WM/desktop
+DISPLAY=:0 xprop -root _NET_SUPPORTING_WM_CHECK    # 'not found' = no window manager
+```
+
+Fix: run a window manager on `:0`. The installer ships a `headless-desktop.service`
+(Openbox) for exactly this and enables it whenever it detects `capture = x11`:
+
+```bash
+systemctl --user enable --now headless-desktop.service
+DISPLAY=:0 xprop -root _NET_SUPPORTING_WM_CHECK    # now reports a 'window id'
+```
+
+`status.sh` checks for this directly: with `capture = x11` it now FAILs if no
+window manager is present on `:0`, instead of only verifying the X server answers.
+
 ---
 
 ## Custom keybinding to escape Moonlight (Mac)