tests: cover Vulkan RTX world extraction

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Risky behavior now covered

• Vulkan RTX world BLAS CPU extraction now has deterministic unit coverage for supported BSP face and triangle-soup surfaces.
• The tests cover malformed/unsupported surfaces being skipped, primitive count capping, cap-limited packing, and stable vertex/index emission order.

Test files added/updated

• Added tests/unit/test_vk_rtx_world.c.
• Registered unit_vk_rtx_world in CMakeLists.txt with USE_VULKAN_RTX=1.
• Updated tests/README.md so the new target is discoverable.

Why this materially reduces regression risk

• RTX BLAS extraction is renderer-critical and feeds acceleration-structure geometry; off-by-one cap handling or bad surface parsing can produce missing geometry, corrupt ray hits, or GPU-side faults.
• The new test exercises the production helper directly without requiring Vulkan hardware or game data, keeping coverage deterministic for CI/headless agents.

Validation

• cmake -S . -B build-coverage-tests -DBUILD_TESTING=ON -DBUILD_UNIT_TESTS=ON -DUSE_VULKAN_RTX=ON && cmake --build build-coverage-tests --target unit_vk_rtx_world
• ctest --test-dir build-coverage-tests -R unit_vk_rtx_world --output-on-failure
• cmake --build build-coverage-tests --target unit_vm_native_module unit_vk_rtx_world && ctest --test-dir build-coverage-tests -R 'unit_(vm_native_module|vk_rtx_world)' --output-on-failure