18 shadow tests added, all 648 tests pass

[github-actions]

Done. Created src/engine/graphics/shadow_tests.zig with 18 tests covering:

Tests Added:

• ShadowCascades.initZero creates identity matrices and zero arrays
• ShadowCascades.isValid returns true for properly initialized cascades
• ShadowCascades.isValid returns false for non-increasing splits
• ShadowCascades.isValid returns false for non-positive texel size
• ShadowCascades.isValid returns false for NaN in splits
• ShadowCascades.isValid returns false for NaN in texel sizes
• computeCascades returns zero cascades for resolution == 0
• computeCascades returns zero cascades for far <= near
• computeCascades returns zero cascades for near <= 0
• computeCascades uses fixed splits for large shadow distance (>500)
• computeCascades uses logarithmic splits for small shadow distance (<=500)
• ShadowConfig default values
• ShadowParams struct layout and defaults
• IShadowScene interface delegation
• CASCADE_COUNT is 4
• getShadowMapHandle returns 0 for out-of-bounds cascade index
• validateCascades returns true for valid cascades
• validateCascades returns false for invalid cascades

All 648 tests pass. The warning "Invalid shadow cascade data detected:" is expected behavior from validateCascades when testing with invalid input.

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