test: cover Vulkan RTX device feature gating

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Summary

• Add a deterministic source-level regression test for Vulkan RTX device feature gating in vk_instance.c.
• Register test_vulkan_rtx_device_features with CTest under unit;scripts;validation;renderer.

Risky behavior now covered

• USE_VULKAN_RTX device creation remains opt-in via r_rtx and requires all KHR extension prerequisites plus vkGetPhysicalDeviceFeatures2.
• RT extensions are only appended after buffer device address, acceleration structure, and ray tracing pipeline feature bits are verified.
• The RTX VkPhysicalDeviceFeatures2 pNext chain wraps existing device pNext state correctly and avoids duplicate debug buffer-device-address chaining.

Test files added/updated

• Added tests/scripts/test_vulkan_rtx_device_features.sh.
• Updated CMakeLists.txt to register the new CTest.

Why this reduces regression risk

• The RTX enablement path is GPU/driver-dependent and difficult to exercise in headless CI, but mistakes can break Vulkan device creation or partially enable ray tracing on unsupported drivers.
• This test pins the critical invariants without requiring RT hardware or game data.

Validation

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