Base R's round() uses "round half to even" (banker's rounding): round(0.5)
is 0, round(2.5) is 2. Stata's round() always rounds a .5 away from
zero: round(0.5) is 1, round(2.5) is 3. Several disclosure-control
calculations in this package (e.g. approx_median()'s "center" position) are
defined in terms of Stata's convention, so this helper is used instead of
base round() wherever the source Stata code calls round().