test: cover Vulkan Forward+ resource guards

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Summary

• Add test_vulkan_forward_plus_guards to pin high-risk Vulkan Forward+ SSBO/resource lifecycle invariants in headless CI.
• Register the guard in CTest and document it in tests/README.md.

Risky behavior now covered

• Forward+ descriptor layout/binding visibility for compute tile cull and PBR fragment reads.
• Real-vs-dummy SSBO graphics descriptor fallback when Forward+ is disabled or partially initialized.
• Render-resolution tile SSBO resize gates, failure handling that preserves the old buffer, and descriptor refresh order.
• Light packing capacity, upload/dispatch barriers, luminance-sort toggle, teardown/pool-reset ordering, r_forwardPlusShade world-pipeline-only invalidation, and RB_DrawSurfs ensure/update/upload/dispatch ordering.

Test files added/updated

• Added: tests/scripts/test_vulkan_forward_plus_guards.py
• Updated: CMakeLists.txt
• Updated: tests/README.md

Why these tests materially reduce regression risk

Forward+ resource setup is GPU-dependent and difficult to execute deterministically in this headless environment, but regressions here can cause stale descriptors, invalid SSBO bindings, black-screen pipeline teardown, or render-scale resize failures. The new guard protects the key source-level invariants without requiring Vulkan hardware.

Validation

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