27 shadow system tests added for graphics/shadows

[MichaelFisher1997]

Tests committed successfully. Summary of tests added:

src/engine/graphics/shadow_tests.zig - 27 tests covering:

• ShadowSystem init/deinit: rejects zero resolution, accepts valid resolutions, initializes state correctly
• ShadowCascades: initZero() defaults, isValid() edge cases (non-finite splits, zero texel size, non-increasing splits, non-finite matrices, zero split)
• computeCascades: invalid input handling (zero resolution, far==near, near<=0), split ordering, positive texel sizes, fixed vs logarithmic split selection based on shadow distance, reverse-Z vs standard-Z, extreme sun directions
• ShadowUniforms: struct size and field offset layout
• IShadowContext: getShadowMapHandle bounds checking (returns 0 for out-of-bounds cascade)
• ShadowConfig/ShadowParams: default values and struct layout

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