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/plugins/extractors/tap-uk-ees-destinations/tests/test_tap.py

@@ -132,3 +132,68 @@ def test_establishment_dimensions_are_not_pinned():
     establishment_totals = {"4369U", "rgHcN", "EfHQq", "S4ROV"}
     assert not set(KS4_PINNED) & establishment_totals
     assert not set(KS5_PINNED) & establishment_totals
+
+
+# ── Grain separation ────────────────────────────────────────────────────────
+#
+# School rows and the England reference were originally one stream with a
+# nullable `urn` in the primary key. target-postgres turns primary_keys into a
+# NOT NULL constraint, so the first national row killed the loader mid-run and
+# the tap saw only a BrokenPipeError on its stdout — a symptom several frames
+# away from the cause.
+
+def _streams():
+    from tap_uk_ees_destinations.tap import TapUKEESDestinations
+    return TapUKEESDestinations(config={}, validate_config=False).discover_streams()
+
+
+def test_no_stream_has_a_nullable_primary_key_column():
+    for stream in _streams():
+        props = stream.schema["properties"]
+        for key in stream.primary_keys:
+            assert key in props, f"{stream.name}: key {key} is not in the schema"
+        if "urn" in stream.primary_keys:
+            assert stream._level == "SCH", (
+                f"{stream.name} keys on urn but does not emit school rows"
+            )
+
+
+def test_national_streams_carry_no_urn_column_at_all():
+    for stream in _streams():
+        if not stream.name.endswith("_national"):
+            continue
+        assert "urn" not in stream.schema["properties"], (
+            "a school identifier that is null in every row is a grain "
+            "mismatch, not a column"
+        )
+        assert "urn" not in stream.primary_keys
+
+
+def test_national_post_process_drops_the_null_urn():
+    from tap_uk_ees_destinations.tap import KS4NationalDestinationsStream, TapUKEESDestinations
+    tap = TapUKEESDestinations(config={}, validate_config=False)
+    stream = KS4NationalDestinationsStream(tap)
+    row = {"urn": None, "time_period": "202223", "pupil_group": "all",
+           "destination_measure": "school_sixth_form", "cohort_pupils": "1",
+           "pupils_raw": "1", "percentage_raw": "1"}
+    assert "urn" not in stream.post_process(dict(row))
+
+
+def test_school_and_national_streams_exist_for_both_phases():
+    names = {s.name for s in _streams()}
+    assert names == {
+        "ees_ks4_destinations", "ees_ks5_destinations",
+        "ees_ks4_destinations_national", "ees_ks5_destinations_national",
+    }
+
+
+def test_each_stream_queries_its_own_geographic_level_with_its_own_pins():
+    """The national pass needs establishment pinned to Total; the school pass
+    must not pin it at all, or every school returns zero rows."""
+    for stream in _streams():
+        if stream.name.endswith("_national"):
+            assert stream._level == "NAT"
+            assert stream._pinned_for_level is stream._national_pinned
+        else:
+            assert stream._level == "SCH"
+            assert stream._pinned_for_level is stream._pinned