Add VM native module loader regression coverage

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Summary

This PR adds focused unit-test coverage for native VM module fallback name resolution introduced in recent VM loader changes, with a minimal testability refactor that preserves runtime behavior.

Risky behavior now covered

• Native VM module filename candidate ordering used by VM_TryLoadNativeModule:
  1. module.so
  2. module.<arch>.so
  3. module<arch>.so
• Input validation behavior for candidate generation (invalid index, null/empty inputs, invalid output buffer).

These paths are high blast-radius because they affect dynamic loading for core modules (game/cgame/ui aliases) and fallback behavior across architectures.

Test files added/updated

• Added: tests/unit/test_vm_native_loader.c
• Added: src/qcommon/vm_native_loader.c
• Added: src/qcommon/vm_native_loader.h
• Updated: src/qcommon/vm.c (uses extracted helper in existing load flow)
• Updated: CMakeLists.txt (registers unit_vm_native_loader target/test)

Why this materially reduces regression risk

• Locks down the exact filename fallback sequence expected by recent production logic changes.
• Prevents subtle architecture-specific loader regressions (e.g., accidental ordering changes, missing module.<arch>.so support) from shipping unnoticed.
• Provides deterministic, dependency-light unit coverage for a core loading path without requiring renderer or full engine startup.

Validation

• Reconfigured build directory and built the new unit target:
  • cmake -S /workspace -B /workspace/build-vk-Release
  • cmake --build /workspace/build-vk-Release --target unit_vm_native_loader
• Ran focused test target:
  • ctest -R unit_vm_native_loader --output-on-failure
• Result: PASS