fix(scripting): flush Node output before exit

[dawNotPoi]

Fixes VoidenHQ/voiden#554

Summary

• wait for the piped stdout write to finish before the Node wrapper exits
• use the same flush-safe exit path for successful, timed-out, and failed worker runs
• add regression coverage for a 4 MiB result and non-zero failure exit codes

Testing

• npm test
• npm run build
• node build-main.mjs
• node build-runner.mjs