test: guard native module loading regressions

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Summary

• Add test_native_module_regressions to guard native VM call argument handling and pk3-backed native library extraction/cache invariants.
• Register the new script with CTest and document it in tests/README.md.

Risky behavior now covered

• Native VM_Call zero-fills omitted vmMain args before copying varargs and passes three stable native slots.
• Native module alias fallback order stays compatible for legacy/project module names.
• pk3-backed native libraries remain cvar-gated, extracted by basename under vm/native_cache/, cache-reused only after length/CRC validation, and tried before loose filesystem paths.

Test files added/updated

• Added tests/scripts/test_native_module_regressions.sh
• Updated CMakeLists.txt
• Updated tests/README.md

Why this reduces regression risk

This path affects native game/cgame/ui module startup and full-conversion compatibility. The guards pin security- and compatibility-sensitive behavior that is hard to exercise in headless CI, especially pk3 extraction/cache ordering and native vararg handling.

Validation

• tests/scripts/test_native_module_regressions.sh
• cmake -S . -B build-coverage-tests -DBUILD_TESTING=ON -DBUILD_UNIT_TESTS=OFF && ctest --test-dir build-coverage-tests -R test_native_module_regressions --output-on-failure