A one-row data.frame with columns term (always
"smd"), estimate (the point estimate of d),
ci_lower (the lower confidence limit on \(\delta\)),
ci_upper (the upper confidence limit on \(\delta\)), and
conf_level (the confidence level used in construction; NA
when asymmetric alpha_lower / alpha_upper were supplied
instead). Column names follow the broom convention so the result
composes with the broom ecosystem.