Seven TDD tasks. Records the two measured facts the design rests on: aggregate
IG rows carry no harmonized_code (so the IG leg must COALESCE item_code), and
aggregate IG codes are year-disjoint from their modern leaf components (so
skipping NOT is_aggregate cannot double-count). Baseline measured at PASS 467.
Plan lives under .superpowers/ because docs/ is the gitignored pkgdown output
dir; .Rbuildignore'd so it never ships in the package tarball.
jared