examples/guest_c: 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 small Node.js ABI smoke test for the C guest example, similar in spirit to the existing Zig guest test.

Why

• non-Rust guest examples benefit from automated ABI checks
• this catches import/export regressions early
• it improves confidence for contributors from C/WebAssembly backgrounds

Scope

• add examples/guest_c/test.mjs
• verify key ABI expectations (imports/exports and minimal runtime behavior)
• document how to run the test in the example README

Done when

• test runs successfully against built guest.wasm
• ABI expectations are explicit and stable
• README includes test command

Notes

Please keep the test lightweight and deterministic.

Thanks again for considering contributing to Numax.