tests: cover Vulkan mesh shader opt-in

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Summary

• Add a deterministic regression test for optional VK_NV_mesh_shader enablement.
• Register the test with CTest under unit;scripts;validation;renderer.

Risky behavior now covered

• r_vk_meshShaderNV must remain default-off, latched, range-limited, and documented as requiring vid_restart.
• Device extension enablement must require driver support, explicit cvar opt-in, and extension-list capacity.
• vk.meshShaderNV must track enablement and the feature chain must request mesh shaders only while leaving task shaders disabled.

Test files added/updated

• Added tests/scripts/test_vulkan_mesh_shader_opt_in.sh
• Updated CMakeLists.txt

Why this reduces regression risk

This protects an experimental hardware/driver-facing Vulkan extension path from accidentally becoming enabled by default or bypassing explicit opt-in. The test is source-invariant based, so it runs deterministically in headless CI without requiring Vulkan mesh-shader-capable hardware.

Validation

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