Refactors `Color` to track both exposed and stored alpha representations, adds `ToScaledVector4(PixelAlphaRepresentation)`, and adjusts pixel conversion paths so associated formats preserve canonical values without unnecessary unpremultiply/reassociate loss. This also updates equality/hash behavior to compare canonical scaled values.
SIMD helpers are split into `MultiplyAddEstimate` vs `FusedMultiplyAdd`, with byte-to-float normalization updated to match scalar rounding exactly across vector widths. JPEG converters, resize kernels, and Porter-Duff/associated-alpha blending paths were updated to use the appropriate helper for either fast estimate or strict fused rounding semantics. Tests were expanded substantially (including exhaustive component/alpha cases and fused-order checks), incidental test cleanup was applied, benchmark comment snapshots were refreshed, and one black/white reference output image was updated.
James Jackson-South
4b07920c7c