test: guard Forward+ pipeline invalidation

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Risky behavior now covered

• r_forwardPlusShade changes must invalidate only Vulkan world graphics pipelines.
• vk_destroy_world_graphics_pipelines() must destroy GPU pipeline handles while preserving vk.pipelines_count and pipeline definitions so cached shader_t::vk_pipeline indices do not point at wrong or empty rows.
• The frame-begin path must not call broad vk_destroy_pipelines() for Forward+ shade changes, avoiding unrelated post-process pipeline teardown and black-screen regressions.

Test files added/updated

• Updated tests/scripts/test_vulkan_regression_source_guards.sh
• Updated tests/README.md

Why these tests materially reduce regression risk

• The covered path is headless-hard to exercise but has high visual blast radius: a small invalidation mistake can corrupt or blank world rendering after a cvar toggle.
• The guard checks the exact invariants from the recent fix: preserve cached pipeline indices/definitions, clear only destroyed handles, and keep r_forwardPlusShade scoped to world pipeline invalidation.

Validation

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