test: cover shader compilation script regressions

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Risky behavior now covered

• scripts/compile_shaders.sh argument validation for --generated-dir
• mocked shader compilation through generated output routing
• RTX demo SPIR-V embed generation for rgen/rmiss/rchit blobs
• Forward+ compute shader binding emission
• --apply replacement behavior with backups of previous tracked shader blobs
• cleanup of temporary .spv files

Test files added/updated

• Added tests/scripts/test_compile_shaders_regressions.sh
• Registered test_compile_shaders_regressions in CMakeLists.txt
• Documented the test in tests/README.md

Why this reduces regression risk

The shader compilation script sits on the renderer build path and recently gained/changed RTX embed and generated-shader behavior. The new fixture test runs the script end-to-end without requiring Vulkan SDK tools or GPU hardware, so CI can catch script regressions before they corrupt generated shader blobs or omit embedded RTX shader data.

Validation

• tests/scripts/test_compile_shaders_regressions.sh scripts/compile_shaders.sh
• cmake -S . -B build-coverage-tests -DBUILD_TESTING=ON -DBUILD_UNIT_TESTS=OFF && ctest --test-dir build-coverage-tests -R test_compile_shaders_regressions --output-on-failure
• ctest --test-dir build-coverage-tests -R 'test_compile_(engine_lto|shaders_regressions)' --output-on-failure