Add pk3 native library cache regression coverage

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Summary

• Adds test_pk3_native_library_cache to guard pk3-backed native module extraction/cache invariants in src/qcommon/files.c.
• Registers the script with CTest and updates tests/README.md.

Risky behavior now covered

• com_nativeLibraryExtractPk3 remains archived, default-on, described, and logged at startup.
• pk3 native-module loading remains gated by the kill-switch, filters .so/.dll inputs, caches by basename under vm/native_cache/, probes direct/vm//modules/ pk3 paths in order, rejects implicit fallbacks for explicit paths, and loads cached files only after size and CRC checks.
• FS_LoadLibrary continues trying the pk3 cache before loose filesystem paths.

Test files added/updated

• Added tests/scripts/test_pk3_native_library_cache.sh
• Updated CMakeLists.txt
• Updated tests/README.md

Why this reduces regression risk

This path has filesystem side effects, native loader behavior, CRC reuse checks, and mod-loading compatibility impact. The source-guard test is deterministic and headless-friendly while pinning the invariants most likely to regress during future loader or filesystem edits.

Validation

• tests/scripts/test_pk3_native_library_cache.sh
• cmake -S . -B build-coverage-tests -DBUILD_TESTING=ON -DBUILD_UNIT_TESTS=OFF && ctest --test-dir build-coverage-tests -R test_pk3_native_library_cache --output-on-failure