Move the WSL / /dev/dxg / d3d12_dri.so probes into _prefer_wsl_d3d12 with
the probed paths as module constants, so the launcher call site is a single
line and a test can lay out a fake WSLg host without touching real
/dev or /usr/lib. The two tests now run the real _desktop_launch_env end to
end (selected under WSL+dxg+driver; untouched with an explicit Mesa
override, off WSL, without /dev/dxg, or without the driver file) instead of
unit-testing the helper with a precomputed boolean.
Docs: one paragraph in the Desktop guide on the automatic selection and
the env vars that keep an explicit choice authoritative.
Follow-up to Xipong's fix for #106117 (salvaged from #106118).
teknium1