fix(dags): build stg_gias_ukprn monthly, never daily (review)

Rebuilding the view in the daily build dropped the Ofsted views on it
(CASCADE), which dim_school reads: every daily run would have failed. A
selector test now rejects any build that drops views it then reads.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pipeline/transform/models/staging/stg_gias_ukprn.sql

@@ -3,6 +3,9 @@
 -- A model on the raw source rather than a column of stg_gias_establishments:
 -- a ref to that model would put the Ofsted models into the daily build's
 -- stg_gias_establishments+ selection (pipeline/tests/test_dag_selectors.py).
+-- Built by the monthly Ofsted DAG with the views on it, never by the daily
+-- one: rebuilding a view drops the views on it (CASCADE), and dim_school, which
+-- the daily build makes, reads int_ofsted_latest and int_ofsted_listed.
 
 select
     cast("URN" as integer)          as urn,