fix: cog_categories() surfaces balance subtypes and accepts type = "balance"

The balance work added category_type = "balance" rows to the corpus and
cog_balances() to read them, but left cog_categories() -- the discovery
surface -- unable to describe them:

- subtype COALESCEd only spend_subtype and revenue_subtype, so every balance
  row came back with subtype = NA
- type rejected "balance", so there was no way to ask for the holdings
  taxonomy at all

Both matter downstream: cog-api derives its subtype vocabulary from
cog_categories(), so an NA subtype becomes an unusable API parameter. Found
while implementing cog-api#26.

Note cog_balances() itself still takes no subtype argument -- for holdings
category is a strict coarsening of balance_subtype -- but the value belongs
in the discovery surface regardless.

Tests read the expected subtype set independently from the crosswalk parquet
rather than from the function under test.

man/cog_categories.Rd

@@ -7,8 +7,8 @@
 cog_categories(type = NULL, pattern = NULL)
 }
 \arguments{
-\item{type}{Either `NULL` (default, return both spending and revenue
-rows), `"spending"`, or `"revenue"`.}
+\item{type}{Either `NULL` (default, every row: expenditure, revenue and
+balance), `"spending"`, `"revenue"`, or `"balance"`.}
 
 \item{pattern}{Optional regex matched case-insensitively against the
 `category` column (e.g. `"Police"` or `"Tax"`).}
@@ -22,7 +22,15 @@ Tibble with columns `category`, `category_type`, `subtype`,
 Returns the category taxonomy exposed by the corpus's
 `summary_categories` view, grouped to one row per
 `(category, subtype)` pair. Use this to discover valid `category`
-values for [cog_spending()] / [cog_revenue()] /
+values for [cog_spending()] / [cog_revenue()] / [cog_balances()] /
 [cog_geographic_rollup()] and to audit which Census item codes feed
 each category.
 }
+\details{
+`subtype` COALESCEs the crosswalk's three subtype columns, so it carries
+`spend_subtype` on expenditure rows, `revenue_subtype` on revenue rows and
+`balance_subtype` on balance rows. Note that [cog_balances()] itself takes
+no `subtype` argument — for holdings, `category` is a strict coarsening of
+`balance_subtype` — but the value is surfaced here because it is the
+discovery surface downstream consumers build their vocabulary from.
+}