Add minimal window diagnostics

[Sunrisepeak]

Summary

• Add a minimal module-only GLFW/OpenGL window example.
• Expose GetError through the GLFW backend wrappers so examples can print actionable GLFW failure details.
• Build the minimal window example in CI and document the runtime boundary.

Test plan

• mcpp test
• cd examples/basic && mcpp run
• cd examples/minimal_window && mcpp build
• cd examples/glfw_opengl3 && mcpp build
• git diff --check