feat(pipeline): add fact_ofsted_latest, one current Ofsted status per school

The backend will read this instead of picking the latest row of fact_ofsted_inspection itself (twice, with arbitrary ties). Schema tests and assert_ofsted_current_grade_consistent pin its invariants. Built by the monthly Ofsted DAG (int_ofsted_latest+), not the daily one.

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

pipeline/transform/tests/assert_ofsted_current_grade_consistent.sql

@@ -0,0 +1,10 @@
+-- A grade is dated by the inspection that awarded or confirmed it, never
+-- after the latest visit, and has a date and a basis exactly when it exists.
+-- A report card leaves no legacy grade in force.
+
+select urn
+from {{ ref('fact_ofsted_latest') }}
+where current_grade_date > latest_visit_date
+   or (current_grade is null) <> (current_grade_date is null)
+   or (current_grade is null) <> (current_grade_basis is null)
+   or (latest_visit_kind = 'report_card' and current_grade is not null)