Calculate total emissions (robust and boundary-aware)
Source:R/calc_total_emissions.R
calc_total_emissions.RdAggregates results from different sources (enteric, manure, soil, energy, inputs)
even if they don't use exactly the same field name for the total.
IMPORTANT: If a source explicitly reports co2eq_kg = NULL (e.g. excluded by
system boundaries), it is treated as zero and no fallback summation is attempted.