Function phase_linearity_triplet 

fn phase_linearity_triplet(a: f64, b: f64, c: f64) -> f64

Phase linearity of three consecutive phase values.

max(0, 1 - |wrap(a - 2b + c) / 2|) — measures how linear the phase is. Matches ROMEO.jl phaselinearity(P, i, j, k).