recode MICS sibling variables to the conventions the package expects
recode_mics_sib_vars.RdMICS and the DHS code survival status differently, and the difference is
silent: MICS MM16 is 1 yes / 2 no / 8 don't know, while the DHS mm2 is
1 alive / 0 dead. Everything downstream filters on sib.alive %in% c(0,1),
so passing MICS codes through unchanged drops every dead sibling as
though its survival status were missing – which silently drives every
mortality estimate to zero.