Function interpolate_75_25 

fn interpolate_75_25(p0: PremulRgba8, p1: PremulRgba8) -> PremulRgba8

Blend 2 RGBA pixels using [0.75, 0.25] weights.

Computes 0.75×p0 + 0.25×p1 using efficient integer arithmetic. Used during upsampling for positions closer to the first pixel. Adds 2 before the shift to implement round-to-nearest instead of floor division.