Add regression tests for VM native module filename fallbacks

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Summary

• add focused unit coverage for VM native module native-library filename fallbacks introduced in recent VM loader changes
• extract native module candidate filename formatting into a tiny helper (src/qcommon/vm_native_module_names.c/.h) without changing loader behavior
• wire new unit test into CTest (unit_vm_native_module_names)

Risky behavior now covered

• exact fallback candidate ordering used by native VM loading:
  1. module.so
  2. module.arch.so
  3. modulearch.so
• invalid candidate index handling (must produce empty output)
• defensive behavior for null inputs and tiny output buffers (no stale output / deterministic empty string)

Test files added/updated

• Added: tests/unit/test_vm_native_module_names.c
• Added: src/qcommon/vm_native_module_names.c
• Added: src/qcommon/vm_native_module_names.h
• Updated: src/qcommon/vm.c (uses helper, behavior-preserving refactor)
• Updated: CMakeLists.txt (register new unit target/test)

Validation run

• cmake -S /workspace -B /workspace/build-vk-Release
• cmake --build /workspace/build-vk-Release --target unit_vm_native_module_names unit_macros -j4
• ctest -R "unit_(macros|vm_native_module_names)" --output-on-failure -V (passed)

Why this materially reduces regression risk

Native VM loading is a core startup path with high blast radius: if fallback filename formatting/order regresses, game/client/ui modules can fail to load at runtime. These tests lock the exact candidate contract and edge-condition behavior, catching subtle naming/fallback regressions early without requiring platform-specific runtime setups.