Function build_segments 

fn build_segments(
    nx: u32,
    ny: u32,
    params: MotionVectorParams,
    warp_field: Option<WarpField<'_>>,
    vertices: &mut Vec<MotionVectorVertex>,
)

Build the per-frame segment list. Pulled out as a free function so it can be unit-tested without a GPU device. When warp_field is Some, segments point along the local warp displacement; without it the function falls back to a horizontal stub.