Set up the first CMake skeleton

[kio-png]

The repository needs a small build skeleton before adding real Box3D wrappers.

For the first pass, I want just enough structure to build an example and a smoke test without pulling in too much project machinery.

Things to add

• root CMakeLists.txt
• header-only box3cpp target
• include/box3cpp/ directory
• one tiny example
• one smoke test
• basic GitHub Actions build

This should stay boring and minimal. The goal is to make the project buildable before adding World, Body, or Shape.