[codex] add X11-backed compat runtime packages

[Sunrisepeak]

Summary

• add Linux X11/XCB/X extension compat runtime packages that build upstream sources into libX11.so, libxcb.so, and related libX*.so libraries
• extend compat.glfw from null-only to Linux X11 plus macOS/Windows backend descriptors, and add compat.khrplatform for OpenGL header closure
• add smoke tests and a Linux CI smoke job that builds upstream-style consumers through a local mcpp path index

Validation

• git diff --check
• release asset mcpp 0.0.38 full Linux smoke:
  • tests/smoke_compat_core.sh
  • tests/smoke_compat_imgui.sh
  • tests/smoke_compat_archive.sh
  • tests/smoke_compat_imgui_window.sh
• local source-built mcpp 0.0.38 hidden-window smoke with MCPP_INDEX_RUN_WINDOW_SMOKE=1 tests/smoke_compat_imgui_window.sh
• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 pytest -q tests/m/test_mcpp.py in openxlings/xim-pkgindex still passes for the published mcpp@0.0.38 package index entry

Notes

The CI smoke uses the published Linux mcpp 0.0.38 release artifact. The window smoke runs only the build/link path in CI; runtime GLX/OpenGL execution remains host-dependent and is gated behind MCPP_INDEX_RUN_WINDOW_SMOKE=1 for local machines with a usable DISPLAY and GL driver stack.

macOS and Windows GLFW descriptor sections are included, but this PR still needs CI or runner coverage on those platforms before claiming full cross-platform runtime verification.