setup_complement_tag: Setup the complement-count tag, in the same shape as a region tag so
it shares one makeContent.EulerTags() pass.
Description
Acts as a quantity-only tag with combo_key = "". The complement
label text comes either from complement$label (user override) or
from container_data$quantity_label (the fitted complement count).
Usage
setup_complement_tag(container_data, complement, number)