Function interpolate_25_75 

fn interpolate_25_75(p0: PremulRgba8, p1: PremulRgba8) -> PremulRgba8

Blend 2 RGBA pixels using [0.25, 0.75] weights (right-weighted interpolation).

Used for upsampling to generate the second of two output pixels. Favors the right/next pixel (p1) with 75% weight. Adds 2 before the shift to implement round-to-nearest instead of floor division.