Add native module regression coverage

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Risky behavior now covered

• Native VM_Call must zero-fill missing vmMain arguments before invoking native entry points, preventing nargs=0 calls from passing stack garbage.
• Pk3-backed native module extraction must keep its archived/default-on toggle, basename-only cache path, direct/vm/modules fallback order, CRC-checked cache reuse, direct Sys_FOpen/fwrite cache writes, and cache-before-loose FS_LoadLibrary ordering.

Test files added/updated

• Added tests/scripts/test_native_module_regressions.sh.
• Registered test_native_module_regressions in CMakeLists.txt.
• Documented the script in tests/README.md.

Why this reduces regression risk

These guards cover recent native module fixes with crash/security/loading blast radius, without requiring game data, a display server, or a GPU. They make the intended invariants explicit in CTest so future edits to VM calls or pk3 native library extraction fail deterministically.

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