feat(legacy-ks2): add stream for pre-COVID KS2 data (2015-2019)

- Add LegacyKS2Stream to tap-uk-ees: downloads old DfE england_ks2final.csv
  files from a configurable base URL, maps 318-column wide format to the
  same schema as stg_ees_ks2 output
- Add stg_legacy_ks2.sql staging model with safe_numeric casts
- Add legacy_ks2 source to _stg_sources.yml
- Update int_ks2_with_lineage.sql to union EES + legacy data
- Configurable via legacy_ks2_base_url and legacy_ks2_years tap settings

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pipeline/transform/models/intermediate/int_ks2_with_lineage.sql

@@ -1,7 +1,14 @@
 -- Intermediate model: KS2 data chained across academy conversions
 -- Maps predecessor URN data to the current active URN
+-- Unions EES (2022+) and legacy (2015-2019) school-level data
 
-with current_ks2 as (
+with all_ks2 as (
+    select * from {{ ref('stg_ees_ks2') }}
+    union all
+    select * from {{ ref('stg_legacy_ks2') }}
+),
+
+current_ks2 as (
     select
         urn as current_urn,
         urn as source_urn,
@@ -15,7 +22,7 @@ with current_ks2 as (
         rwm_expected_boys_pct, rwm_high_boys_pct, rwm_expected_girls_pct, rwm_high_girls_pct,
         rwm_expected_disadvantaged_pct, rwm_expected_non_disadvantaged_pct, disadvantaged_gap,
         disadvantaged_pct, eal_pct, sen_support_pct, sen_ehcp_pct, stability_pct
-    from {{ ref('stg_ees_ks2') }}
+    from all_ks2
 ),
 
 predecessor_ks2 as (
@@ -33,11 +40,11 @@ predecessor_ks2 as (
         ks2.rwm_expected_boys_pct, ks2.rwm_high_boys_pct, ks2.rwm_expected_girls_pct, ks2.rwm_high_girls_pct,
         ks2.rwm_expected_disadvantaged_pct, ks2.rwm_expected_non_disadvantaged_pct, ks2.disadvantaged_gap,
         ks2.disadvantaged_pct, ks2.eal_pct, ks2.sen_support_pct, ks2.sen_ehcp_pct, ks2.stability_pct
-    from {{ ref('stg_ees_ks2') }} ks2
+    from all_ks2 ks2
     inner join {{ ref('int_school_lineage') }} lin
         on ks2.urn = lin.predecessor_urn
     where not exists (
-        select 1 from {{ ref('stg_ees_ks2') }} curr
+        select 1 from all_ks2 curr
         where curr.urn = lin.current_urn
           and curr.year = ks2.year
     )