fix(build): gate V8 nullptr_t compiler workaround

[ilysenko]

Summary

• add a reusable V8 nullptr_t compiler workaround helper that probes with before wrapping CXX
• use the helper for both the install native-module rebuild path and the Nix native-modules build
• add smoke coverage for both the no-workaround and wrapper-needed cases

Validation

• bash -n scripts/lib/native-modules.sh tests/scripts_smoke.sh
• git diff --check
• ./tests/scripts_smoke.sh

Supersedes #303.