Tier 0 — Root sources (no deps): cp, metaregion, npl, sna, income_groups, indicators, dictionary, nan, pfw, wdi, maddison, country_list
Tier 1 — ppp, pop, cpi, regions, metadata, weo, gdm, countries
Tier 3 — gdp (composite, write→GH→read)
Tier 4 — missing_data (deepest, 12 upstream inputs)