tests: cover native module pk3 loading guards

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Risky behavior now covered

• pk3-backed native module extraction/cache path controlled by com_nativeLibraryExtractPk3.
• Basename-only native cache placement under vm/native_cache/, direct/vm/modules fallback order, explicit-path no-fallback behavior, disk length+CRC cache reuse, direct cache writes, diagnostics, and FS_LoadLibrary probing pk3 cache before loose files.
• Lua debug script loading fallback from luaL_loadfile to FS_ReadFile + luaL_loadbuffer for scripts stored in pk3s.

Test files added/updated

• Added tests/scripts/test_native_module_regressions.sh.
• Registered test_native_module_regressions in CMakeLists.txt.
• Updated tests/README.md validation list.

Why this reduces regression risk

Native module loading is a core mod/full-conversion path, and failures can silently break pk3-only native UI/game modules. These deterministic guards protect the intended extraction/cache invariants without requiring real game data, native libraries, or dlopen-capable fixtures.

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