docs(news): per-year population denominators (unreleased)

Summarizes the per-capita and peer-cohort behavior changes for users
upgrading from earlier 0.1 snapshots.

NEWS.md

@@ -1,5 +1,42 @@
 # uscogdata 0.1.0 (development)
 
+## Per-capita denominators now use per-year Census F-33 population
+
+* `cog_spending()` and `cog_revenue()` previously divided all years' amounts
+  by a single ACS 2018-2022 estimate (`canonical_fips_xwalk.population_acs`),
+  producing biased per-capita values for time-series analysis. They now
+  divide by the F-33 `population` recorded on each gov-year via the new
+  `gov_population_yearly` view. Result tibbles gain a `pop_source` column
+  with values `"census_f33"` or `"unavailable"`. `notes` is updated to
+  concatenate multiple notes with `"; "`.
+
+## Peer cohorts can be set to a chosen year
+
+* `cog_find_peers()` adds a `year` argument (default: most recent year for
+  which the target has an observed population in `gov_population_yearly`).
+  The returned column previously named `population_acs` is now `population`
+  and reflects the cohort year's vintage. The cohort year is attached to the
+  returned tibble as `attr(x, "cohort_year")`.
+* `cog_peer_compare()` now stamps a `cohort_year` column on its result (read
+  from the peers tibble's attribute) and records `cohort_year` plus
+  `cohort_govids` in provenance. When the caller supplies a bare character
+  vector instead of a `cog_find_peers()` result, `cohort_year` is `NA`.
+
+## Rollups exclude govs missing population
+
+* `cog_geographic_rollup(per_capita = TRUE)` drops rows whose government has
+  `pop_source == "unavailable"` and records the dropped govids in
+  `provenance$rollup$excluded_govids`. This excludes special districts
+  (type 4) and school districts (type 5) from per-capita rollups by design.
+
+## New: vignette and provenance metadata
+
+* New vignette `population-denominators` covers the four population sources,
+  the type-4/5 coverage gap, the popyear quirk, and how to build moving-window
+  peer cohorts manually.
+* Provenance gains `transformations$per_capita$popyear_range` and
+  `pop_source_counts`. `cog_explain()` renders both.
+
 ## New features
 
 * `cog_gov_search()` gains a **basket mode**: passing vector `name`