fix(pipeline): harden banding against EES sentinels; diagnostic cleanups

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

pipeline/transform/models/marts/_marts_schema.yml

@@ -110,6 +110,7 @@ models:
               values: ['Well above average', 'Above average', 'Average', 'Below average', 'Well below average']
               config:
                 where: "progress_8_banding is not null"
+                severity: warn
       - name: attainment_8_disadvantage_gap
       - name: progress_8_disadvantage_gap
     tests:

pipeline/transform/models/staging/stg_ees_ks4.sql

@@ -63,7 +63,13 @@ info as (
         {{ safe_numeric('sen_pupil_percent') }} as sen_pct,
         {{ safe_numeric('sen_with_ehcp_pupil_percent') }} as sen_ehcp_pct,
         {{ safe_numeric('sen_no_ehcp_pupil_percent') }} as sen_support_pct,
-        nullif(trim(progress8_banding), '') as progress_8_banding,
+        -- EES suppression sentinels (z/c/x/q/u) and blanks must not reach the
+        -- mart as banding labels
+        case
+            when lower(trim(progress8_banding)) in ('', 'z', 'c', 'x', 'q', 'u', 'null')
+                then null
+            else trim(progress8_banding)
+        end as progress_8_banding,
         {{ safe_numeric('attainment8_diffn') }} as attainment_8_disadvantage_gap,
         {{ safe_numeric('progress8_diffn') }} as progress_8_disadvantage_gap
     from {{ source('raw', 'ees_ks4_info') }}