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/FOLLOWUPS.md

@@ -247,6 +247,14 @@ TROUBLESHOOTING.md §13.
 - Debian/Ubuntu package install path (`apt` + the `.deb`), since `yay`/AUR
   don't exist there. This is a larger lift than the flag itself.
 
+**Done so far**: the *desktop* piece of the x11 backend is now reproducible.
+`install.sh` detects `capture = x11` (via `capture_backend_is_x11`) and installs
++ enables `files/headless-desktop.service` (Openbox on `:0`) so the headless
+Xorg has a window manager to render — without it, capture is a black screen.
+`status.sh` gained a matching check (FAIL if no WM on `:0`). Still outstanding:
+the `--backend x11|wlr` flag/auto-detect, the `config.sh` x11 conf variant, and
+shipping the `xorg-headless.service` + `xorg-headless.conf` templates.
+
 **Complexity**: medium-high. The capture-backend split is moderate; full
 Debian packaging support is the bulk of the work.