Under a rootless XWayland session (the GNOME/KDE/sway default) the X11
root window has no backing pixmap, so core `GetImage` on the root returns
`BadMatch`. The computer tool advertised Wayland support yet failed every
screenshot with a raw X11 protocol dump, and coordinate actions stayed
gated behind a capture that could never succeed.
- `Monitor::all` now probes a 1x1 root `GetImage` at initialization and,
on a `Match`/`Drawable` error, fails fast with an actionable
`DESKTOP_BACKEND_UNAVAILABLE` message naming the rootless-XWayland
constraint via the new `root_capture_error` classifier.
- `capture_image` routes its `GetImage` failure through the same
classifier so any surviving path yields the actionable message rather
than a raw protocol dump; unrelated errors stay verbatim.
- Corrected the module doc premise and `docs/computer-use.md` to list
rootless XWayland as unsupported (capture needs a rooted/rootful X
server, which only exposes X11 clients).
Fixes#7085
roboomp
b059550f0b