feat(rollup): drop unavailable-pop rows + provenance audit

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.

R/rollup.R

@@ -8,28 +8,35 @@
 #' "place portraits" that compare a city to the surrounding county and
 #' containing state on one set of axes.
 #'
+#' When `per_capita = TRUE`, rows whose government has no observed
+#' population in that year (`pop_source == "unavailable"`) are dropped from
+#' the result. The dropped govids are recorded in
+#' `provenance$rollup$excluded_govids`. This excludes special districts
+#' (gov type 4) and school districts (gov type 5) from per-capita rollups
+#' by design — see `vignette('population-denominators')`.
+#'
 #' @param govids Named list with any non-empty subset of elements named
 #'   `state`, `county`, `city`. Each element is a character vector of
 #'   `canonical_govid` values. At least one layer required.
 #' @param category Single category name or character vector (passed through
 #'   to [cog_spending()]).
 #' @param years Integer vector of years.
-#' @param per_capita If `TRUE`, per-capita uses each layer's own population
-#'   from `canonical_fips_xwalk.population_acs`.
+#' @param per_capita If `TRUE`, per-capita uses each gov's own per-year
+#'   population from `gov_population_yearly`. Govs with missing population
+#'   are excluded from the result.
 #' @param adjust_to_year Integer base year for CPI-U conversion, or `NULL`.
 #' @return Tibble with columns `year`, `layer`, `canonical_govid`, `gov_name`,
 #'   `spend_subtype`, `category`, `amt_nominal`, optional `amt_real` /
-#'   `amt_per_capita_nominal` / `amt_per_capita_real`, `codes_included`,
-#'   `aggregate_fallback`, `scope_note`, `notes`. Carries a `provenance`
-#'   attribute with `verb = "cog_geographic_rollup"` and `layers`.
+#'   `amt_per_capita_nominal` / `amt_per_capita_real`, optional `pop_source`,
+#'   `codes_included`, `aggregate_fallback`, `scope_note`, `notes`. Carries a
+#'   `provenance` attribute with `verb = "cog_geographic_rollup"`, `layers`,
+#'   and `rollup$included_govids` / `rollup$excluded_govids`.
 #' @export
 cog_geographic_rollup <- function(govids, category, years,
                                   per_capita = FALSE, adjust_to_year = NULL) {
   call <- match.call()
   .validate_rollup_layers(govids)
 
-  # Accept character vector OR a data.frame with canonical_govid per layer,
-  # so cog_gov_search() output can be piped into one of the layer slots.
   govids <- lapply(govids, .coerce_govid_input, arg = "govids[[layer]]")
   if (any(lengths(govids) == 0L)) {
     cli::cli_abort("Each layer in `govids` must be non-empty after coercion.")
@@ -45,12 +52,25 @@ cog_geographic_rollup <- function(govids, category, years,
   r <- dplyr::left_join(r, layer_map, by = "canonical_govid",
                         relationship = "many-to-many")
   r$scope_note <- .rollup_scope_note(r$layer)
+
+  excluded <- character(0)
+  if (isTRUE(per_capita) && "pop_source" %in% names(r)) {
+    drop <- r$pop_source == "unavailable"
+    excluded <- unique(r$canonical_govid[drop])
+    r <- r[!drop, , drop = FALSE]
+  }
+  included <- unique(r$canonical_govid)
+
   r <- .reorder_rollup_cols(r)
 
   prov <- attr(r, "provenance")
   prov$verb   <- "cog_geographic_rollup"
   prov$call   <- paste(deparse(call), collapse = " ")
   prov$layers <- layer_names
+  prov$rollup <- list(
+    included_govids = included,
+    excluded_govids = excluded
+  )
   attr(r, "provenance") <- prov
 
   r