tests: cover native VM module fallback regressions

[cursor]

Risky behavior now covered

• Native VM logical fallback ordering for classic and project-native module names (qagame -> game -> server, cgame -> client, ui -> frontend, and reverse aliases).
• Native vmMain argument packing for direct native VM calls, including zero-filled missing argument slots so zero/one-argument calls do not read stack garbage.

Test files added/updated

• tests/unit/test_vm_native_module.c
• CMakeLists.txt
• tests/README.md

Why this materially reduces regression risk

• Recent native-module compatibility fixes were documented and partially refactored, but the high-risk alias order and native call argument behavior were still embedded in vm.c without direct unit coverage.
• The new tests exercise extracted production helpers deterministically without real native libraries, pk3s, or game data.

Validation

• cmake -S . -B build-coverage-tests -DBUILD_TESTING=ON -DBUILD_UNIT_TESTS=ON
• cmake --build build-coverage-tests --target unit_vm_native_module
• ctest --test-dir build-coverage-tests -R unit_vm_native_module --output-on-failure
• cmake --build build-coverage-tests --target idtech3_server