examples/guest_cpp: add Node-based ABI smoke test

[GianIac]

If you'd like to help with Numax, this is a great place to start, thank you so much for taking a look !

Add a Node.js ABI smoke test for the C++ guest example.

Why

• C++ example documents ABI details but lacks automated verification
• a smoke test lowers risk when build flags/toolchains evolve
• this is a small, educational contribution

Scope

• add examples/guest_cpp/test.mjs
• assert expected imports/exports and basic run behavior through a mock host
• update README with build+test usage

Done when

• test validates ABI shape for guest_cpp
• instructions are documented in example README
• test is easy to run locally

Notes

Please keep assertions focused on stable ABI aspects.

Thanks again for considering contributing to Numax.