Add GLFW GLX runtime closure

[Sunrisepeak]

Summary

• add compat.glx-runtime as a Linux host GLVND/GLX/OpenGL runtime adapter
• make compat.glfw depend on the adapter instead of relying on test-local LD_LIBRARY_PATH shims
• restore standard X11/XCB shared library SONAMEs such as libX11.so.6 and libxcb.so.1
• update validation to use released mcpp 0.0.46

Verification

• bash -n tests/smoke_compat_imgui.sh && bash -n tests/smoke_compat_imgui_window.sh
• lua5.4 -e 'for _,f in ipairs(arg) do local chunk, err = loadfile(f); if not chunk then error(f .. ": " .. err) end end' $(rg --files 'pkgs' -g '*.lua')
• built a temporary minimal ImGui window project with [indices] compat = { path = "/home/speak/workspace/github/mcpplibs/mcpp-index" } using mcpp 0.0.46
• verified a real X11 window with xwininfo: mcpp imgui minimal window

Notes

Local full smoke was not trusted because the workstation root filesystem was full and the shell had an older mcpp 0.0.45 plus ~/.mcpp/registry/bin/xlings 0.4.40; CI should validate on a clean runner with the released mcpp 0.0.46 package.