Uniform-scale-to-fill math made final thickness = target_length /
sum(aspect_ratios), independent of ref_scale. Now ref_scale sets
thickness directly; target_length is a cap (scale down only on
overflow), padding with fill color when the strip is under the cap.
Concatenates a batch of reference images into an identity strip against
a main image, sized to exactly fill the main image's longest side with
no resolution cap, main image never shrunk. Offset controls overlap vs
gap; outputs a mask marking the strip region.
moonwhaler