feat: advertise 'All Categories' from cog_categories()

A reserved value nobody can discover is a trap, and this is the view
the API's /categories endpoint is built from. Emitted for the two flow
vocabularies only -- cog_balances() returns a stock and has no concept
to sum within.

Also fix test-categories.R to exclude pseudo-category rows from
crosswalk-specific assertions (one row per (category, subtype) pair,
non-empty item_codes, valid subtypes).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

man/cog_categories.Rd

@@ -16,7 +16,10 @@ balance), `"spending"`, `"revenue"`, or `"balance"`.}
 \value{
 Tibble with columns `category`, `category_type`, `subtype`,
   `n_codes`, `item_codes` (comma-separated, alphabetical). Sorted by
-  `category_type`, `category`, `subtype`.
+  `category_type`, `category`, `subtype`. Includes one row per flow for the
+  reserved pseudo-category `"All Categories"`, which carries `NA` for
+  `subtype`, `n_codes` and `item_codes` because it is a query mode rather
+  than a crosswalk entry — see [cog_spending()]'s `category` argument.
 }
 \description{
 Returns the category taxonomy exposed by the corpus's