docs: spec for per-year population denominators

Design doc for switching cog_spending / cog_revenue / cog_geographic_rollup
per-capita calculations from a static ACS 2018-2022 population to per-year
F-33 population already present in long.population. Covers shifting peer
matching to a user-selectable cohort year (defaulting to most recent
observed year), type-4/5 NA policy, provenance updates, and a new vignette
enumerating denominator sources for future extensibility.

Specs live in /specs (added to .Rbuildignore) since docs/ is reserved for
pkgdown output.

.Rbuildignore

@@ -10,3 +10,4 @@
 ^\.gitignore$
 \.gitkeep$
 ^vignettes$
+^specs$