cog_geographic_rollup(per_capita = TRUE) now drops rows whose government
has no observed population for that year (pop_source == 'unavailable'),
matching the spec's exclusion rule. Records included/excluded govids in
provenance$rollup.
Wraps cog_spending across a named list of state/county/city layers,
tagging each row with its `layer` and attaching a scope_note that
documents geographic-scope caveats (state totals are statewide, county
totals include areas outside a listed city, city proper excludes
special districts). Per-capita uses each layer's own population from
the canonical_fips_xwalk.
Provenance is inherited from cog_spending but rewritten to reflect
the outer verb (verb, call, layers).
Tests: 19 new / 99 total pass. devtools::check() 0E/0W/2N.
jared