fix(destinations): school rows and the England reference are different grains

The annual DAG died with a BrokenPipeError from Meltano's log writer, which
is several frames from the cause: target-postgres exited first and the tap
saw its stdout close.

The tap declared primary_keys = [urn, ...] while emitting urn=None for the
national rows, and target-postgres turns primary_keys into a NOT NULL
constraint. The first national row of the run failed the insert and took
the loader with it. Every other tap in this repo keys on non-null columns.

Carrying two grains in one stream was the actual mistake, so the fix is to
separate them rather than paper over the null: four streams now, with
ees_ks4/ks5_destinations_national carrying no urn column at all — a school
identifier that is null in every row is a grain mismatch, not a column.
The staging models split the same way and the national mart reads the new
pair instead of filtering `where urn is null`.

Verified against the live API: the school stream yields 135,240 rows over
4,508 schools with no duplicate keys, no null key columns and all 31,382
suppression sentinels intact; the national streams yield 30 and 33 rows
with no urn column.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BvdDKvFFSZuMVDH5fEyTob

pipeline/transform/models/marts/fact_destination_national.sql

@@ -22,8 +22,7 @@ select
     pupils,
     percentage,
     status
-from {{ ref('stg_ees_ks4_destinations') }}
-where urn is null
+from {{ ref('stg_ees_ks4_destinations_national') }}
 
 union all
 
@@ -36,5 +35,4 @@ select
     pupils,
     percentage,
     status
-from {{ ref('stg_ees_ks5_destinations') }}
-where urn is null
+from {{ ref('stg_ees_ks5_destinations_national') }}