feat: register balance_long / balance_annotated behind a column gate (#25)

inst/sql/26-balance_long.sql

@@ -0,0 +1,22 @@
+-- Cash and security holdings, classified by crosswalk MEMBERSHIP on
+-- category_type (see 21-revenue_long.sql for why first-letter prefixes cannot
+-- do this job -- the X and Y families each span revenue, expenditure AND
+-- balance).
+--
+-- These rows are STOCKS: a balance at a point in time, not a flow over a
+-- fiscal year. Summing a stock with a flow is meaningless, which is why they
+-- live behind a third view rather than as a subtype of either money view, and
+-- why neither spending_long nor revenue_long can reach them.
+--
+-- `NOT is_aggregate` mirrors spending_long / revenue_long. The wide-era
+-- aggregate-only holdings codes (X40/X41) are deliberately outside this view;
+-- they are reachable only through the recipe path, which bypasses this filter
+-- by design (cog_pipeline/docs/phase_r_harmonization_review.md § 0.2).
+CREATE OR REPLACE VIEW balance_long AS
+SELECT *
+FROM long
+WHERE item_code IN (
+    SELECT item_code FROM summary_categories
+    WHERE category_type = 'balance'
+  )
+  AND NOT is_aggregate;