docs: document cog_balances() and correct stale CLAUDE.md claims (#25)

Adds the NEWS entry, a Financial data pkgdown reference section (none
existed for cog_spending/cog_revenue), and corrects CLAUDE.md's SQL-layer
claim, view count, test count and fixture-year description against
measured values. Also documents balance_caveats in
inst/schemas/provenance-v1.json (test-first: added a schema-documentation
test to test-balances.R, confirmed it failed, then fixed the schema) and
fleshes out cog_balances()'s @return roxygen to enumerate its conditional
columns, regenerating man/cog_balances.Rd.

tests/testthat/test-balances.R

@@ -359,6 +359,14 @@ test_that("a request past a family's coverage window is flagged", {
   })
 })
 
+test_that("the provenance schema documents balance_caveats", {
+  sch <- jsonlite::fromJSON(
+    system.file("schemas", "provenance-v1.json", package = "uscogdata"),
+    simplifyVector = FALSE
+  )
+  expect_true("balance_caveats" %in% names(sch$properties))
+})
+
 test_that("the caveat message fires once per session", {
   skip_if_no_corpus()
   with_fixture_corpus({