feat(compat): add imgui glfw opengl packages

[Sunrisepeak]

Summary

• add compat.imgui for Dear ImGui v1.92.8 core sources and backend headers
• add compat.glfw for GLFW 3.4 built from source with the null platform backend
• add compat.opengl for Khronos OpenGL API headers, including a generated GL/gl.h shim
• document the new third-party packages and the glfw -> opengl dependency edge

Validation

• Lua syntax: loadfile(..., 't') over all pkgs/*/*.lua
• mcpp smoke: compat.imgui + compat.glfw + compat.opengl build and run with GLFW null platform
• backend smoke: consumer-side build of upstream imgui_impl_glfw.cpp and imgui_impl_opengl3.cpp against the compat packages
• checked generated build.ninja for no -lglfw, no -lGL, and no /usr/include usage