Function upscale 

pub(crate) fn upscale(
    src: &mut Pixmap,
    src_width: u16,
    src_height: u16,
    edge_mode: EdgeMode,
) -> (u16, u16)

Upsample a pixmap by 2x using linear interpolation with [0.75, 0.25] weights.

Uses separable passes: horizontal doubling followed by vertical doubling.

Phase Alignment Theory

The downsampling [1,3,3,1]/8 filter creates a half-pixel offset. Its center of mass is at position 2k + 0.5, not 2k. This means each downsampled pixel at discrete position k represents a sample at continuous position 2k + 0.5.

When upsampling, we reconstruct pixels at positions 2k and 2k+1 from downsampled pixels whose centers are at ..., 2k-1.5, 2k+0.5, 2k+2.5, ...

Interpolation weights are derived from linear interpolation based on distances:

• Position 2k: distance 0.5 from center at 2k+0.5, distance 1.5 from center at 2k-1.5 → weights: 0.75×pixel[k] + 0.25×pixel[k-1]
• Position 2k+1: distance 0.5 from center at 2k+0.5, distance 1.5 from center at 2k+2.5 → weights: 0.75×pixel[k] + 0.25×pixel[k+1]