feat(dbt): fact_ks4_la_averages from DfE's LA rows (H2)

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

docs/ARCHITECTURE.md

@@ -61,6 +61,11 @@ history from the full DataFrame and supplementary data from marts. Comparisons
 batch supplementary queries across selected URNs. Async routes still contain
 synchronous dependency calls; a fully asynchronous database layer is not present.
 
+Benchmarks are DfE's own figures, read from marts: `fact_ks2_national_averages`
+and `fact_ks4_national_averages` for England, and `fact_ks4_la_averages` (all
+state-funded schools) for the search rows' "vs LA avg". The API never averages
+school rows to make a benchmark.
+
 ## Frontend boundaries
 
 `app/(frontend)` owns the public root layout and pages. `app/(payload)` owns the