Extract depth-normalized soil carbon effects

Extract depth-normalized soil carbon effects

For every eligible study, extract soil carbon by depth layer, the bulk density used, the plot counts, and the sampling uncertainty, then normalize each effect to the frozen depth interval. Record studies reporting concentration without bulk density as unresolved. Deliverable: A table with study, depth_layer, effect, standard_error, bulk_density, bulk_density_source, and a normalization flag.