tests: cover Vulkan mesh shader opt-in

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Risky behavior now covered

• Optional VK_NV_mesh_shader device setup remains explicitly gated by driver support, r_vk_meshShaderNV opt-in, and extension-list capacity.
• The experimental cvar remains default-off, latched, and range-limited.
• The Vulkan mesh-shader feature struct preserves the existing pNext chain while enabling only meshShader and leaving taskShader disabled.

Test files added/updated

• Added tests/scripts/test_vulkan_mesh_shader_opt_in.sh
• Registered test_vulkan_mesh_shader_opt_in in CMakeLists.txt

Why this reduces regression risk

• PR Vulkan: optional VK_NV_mesh_shader (r_vk_meshShaderNV); compute/DLSS policy log #85 added opt-in extension/feature negotiation around Vulkan device creation, which has high startup/regression risk and is difficult to exercise in headless CI without specific NVIDIA hardware.
• This deterministic source-level regression test protects the important gating and feature-chain invariants without requiring a GPU or changing production behavior.

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