terminal_aw is start-of-AW biomass in the final observation year.
end_of_year is abundance after that year's seasonal dynamics, converted
with terminal AW weights, selectivity, and legal-size definitions (Bfinal).
Both use mean historical adjusted AW biomass as their OM denominator.
These are distinct quantities. Declaring a target does not convert a fitted
model or train a new response. Unannotated older GP objects retain
unspecified timing until their provenance is supplied explicitly.
Usage
mse_target(reference_years, timing = c("terminal_aw", "end_of_year"))