Required columns of the (required) target sheet that drives
the by-target predictions. A target-request row names one or more
locations (loc_id, a ;-separated list), a target year (year), and an
inclusive age span (age_low..age_high).
A row that names only a loc_id, leaving year/age_low/age_high/dose
blank, inherits those values from the row above (last-observation-carried-
forward), so a run of locations sharing one request need not repeat them.