feat: name a cohort by state/type predicate instead of a 40k-id IN list

cog_spending(), cog_revenue() and cog_balances() gain optional state/type
arguments. Both default to NULL, so every existing govid-based call is
unchanged.

The verbs took a cohort only as a govid vector, which .sql_lit_chr()
rendered into a quoted IN list and .verb_spendrev() embedded into 5-8
separate statements per call: the scope check, the main aggregate, the
per-capita join, the harmonization block, and the suggestion and
suppression queries. For type = "city" that list is 301,589 characters,
parsed and planned from scratch every time it appears.

Passing state/type instead expresses the cohort as a subquery against
canonical_fips_xwalk, so its size never enters the SQL string at all.

Measured on the production corpus, same FY2022 aggregate over the
20,106-government city cohort, DUCKDB_THREADS=2, median of 5:

  IN (20,106 literals) -- 0.3.0            432 ms
  join against a temp cohort table         132 ms
  predicate on canonical_fips_xwalk         102 ms
  no cohort filter at all (the floor)      105 ms

The predicate reaches the no-filter floor: the cohort restriction is
now free. End to end through cog_spending(category = "Police"),
1080 ms -> 271 ms, 3.99x -- larger than the single-query saving,
because the repetition across statements is what actually cost.

Design decisions, both made explicitly rather than left implicit:

  - govid AND state/type INTERSECT. "These ids, narrowed to that
    state/type" is a real query, and an error here could never be
    relaxed later without breaking callers.
  - A predicate cohort has no id list to report, so
    provenance$scope$govids_found/govids_missing stay empty and a new
    scope$cohort block carries state, type and n_governments. Resolving
    the ids just to report them would put 20,000 govids in every
    fleet-scale response body -- the cost this change removes. A
    govid-named cohort's provenance is untouched.

state/type are coerced with .coerce_state_to_fips()/.coerce_type(), the
same helpers cog_gov_search() uses. That is load-bearing: the argument
is a postal abbreviation ("WI") while fips_state holds a FIPS code
("55"), and a predicate on the raw parameter matches nothing and returns
an empty result indistinguishable from "reported nothing". cog-api hit
exactly this trap optimizing the same path.

.attach_per_capita() now keys its population lookup on the govids present
in the result rather than the requested cohort. Those are the only ones
its LEFT JOIN can match, so the output is identical -- but it needs no id
list, and on a paginated call it looks up one page instead of the fleet.

Fixes uscogdata#58.

R/balances.R

@@ -18,7 +18,9 @@
 #' comparable to a GAAP fund balance from an ACFR.
 #'
 #' @param govid Canonical govid(s): a character vector, or a data frame with a
-#'   `canonical_govid` column (e.g. from [cog_gov_search()]).
+#'   `canonical_govid` column (e.g. from [cog_gov_search()]). `NULL` to name
+#'   the cohort by `state`/`type` instead.
+#' @inheritParams cog_spending
 #' @param years Integer vector of fiscal years.
 #' @param category Optional character vector of categories to keep. One of
 #'   `"Fund Balances"`, `"Insurance Trust Balances"`,
@@ -63,15 +65,16 @@
 #'   requested years). `expenditure_concept`/`revenue_concept` are `NA` --
 #'   holdings are a stock, not a flow, so neither concept vocabulary applies.
 #' @export
-cog_balances <- function(govid, years, category = NULL,
+cog_balances <- function(govid = NULL, years, category = NULL,
                          per_capita = FALSE, adjust_to_year = NULL,
-                         basis = c("harmonized", "raw"), recipe = NULL) {
+                         basis = c("harmonized", "raw"), recipe = NULL,
+                         state = NULL, type = NULL) {
   call <- match.call()
   basis <- match.arg(basis, c("harmonized", "raw"))
   # Coerce FIRST, validate second: .validate_verb_inputs() asserts
   # is.character(govid), and a data-frame govid (cog_gov_search() output) has
   # not been unwrapped yet at this point.
-  govid <- .coerce_govid_input(govid)
+  govid <- if (is.null(govid)) NULL else .coerce_govid_input(govid)
   # The money verbs' validator, reused rather than re-implemented (R/spending.R).
   # It covers the exact superset cog_balances() needs -- including the
   # recipe/category mutual-exclusivity guard -- so a second local copy would
@@ -91,6 +94,8 @@ cog_balances <- function(govid, years, category = NULL,
   years <- as.integer(years)
   if (!is.null(adjust_to_year)) adjust_to_year <- as.integer(adjust_to_year)
 
+  cohort <- .make_cohort(govid, state, type)
+
   con <- .ensure_session()
   .require_balance_support(con)
   scope <- .check_govids_in_scope(govid)
@@ -109,7 +114,7 @@ cog_balances <- function(govid, years, category = NULL,
     .validate_recipe_id(con, recipe)
     comps <- .recipe_components(con, recipe)
     recipe_label <- comps$label[[1]]
-    result <- .run_recipe(con, recipe, govid, years)
+    result <- .run_recipe(con, recipe, cohort, years)
     sql <- attr(result, "sql_query")
     result <- .shape_recipe_result(result, "balance_subtype", recipe_label)
     recipe_block <- list(
@@ -119,7 +124,7 @@ cog_balances <- function(govid, years, category = NULL,
     category_for_prov <- recipe_label
   } else {
     sql <- .build_verb_sql("balance_annotated", "balance_subtype",
-                           govid, years, category,
+                           cohort, years, category,
                            ig_view = NULL, subtype_scope = NULL)
     result <- tibble::as_tibble(DBI::dbGetQuery(con, sql))
   }
@@ -127,7 +132,7 @@ cog_balances <- function(govid, years, category = NULL,
   # Order matters (matches .verb_spendrev()): per-capita first, so
   # .attach_real_dollars() deflates the nominal per-capita column into
   # amt_per_capita_real rather than needing amt_per_capita_nominal recomputed.
-  if (isTRUE(per_capita)) result <- .attach_per_capita(result, con, govid)
+  if (isTRUE(per_capita)) result <- .attach_per_capita(result, con)
   if (!is.null(adjust_to_year)) {
     result <- .attach_real_dollars(result, adjust_to_year, per_capita)
   }
@@ -145,6 +150,7 @@ cog_balances <- function(govid, years, category = NULL,
   )
   prov$scope$govids_found   <- scope$found
   prov$scope$govids_missing <- scope$missing
+  prov$scope$cohort <- .cohort_provenance(con, cohort)
 
   prov$balance_caveats <- .balance_caveats(
     con, prov$codes_summed$observed, years