Function projection_axis 

pub fn projection_axis(t: &[[f64; 3]; 3]) -> Option<(DropAxis, Sign)>

PHASE A — pick the drop axis whose projected area is EXACTLY nonzero, plus the reference winding w0 (the orient2d sign of T under that axis, so every output sub-triangle can be emitted with T’s orientation). None ⇒ T is degenerate (zero projected area in every axis).