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.

_pkgdown.yml

@@ -3,6 +3,12 @@ template:
   bootstrap: 5
 
 reference:
+  - title: Financial data
+    desc: Spending, revenue and balance-sheet holdings for one or more governments.
+    contents:
+      - cog_spending
+      - cog_revenue
+      - cog_balances
   - title: Search & basket
     desc: Resolve place names into canonical govids.
     contents: