feat(destinations): serve destinations without closing the gaps

The serialiser carries status through and computes no totals of its own.
The only aggregates in the payload are ones DfE published itself; whether
showing one is safe depends on how many of its components are suppressed,
which the frontend decides.

The batch guard grows from six tables to eight.

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

backend/tests/test_destinations_api.py

@@ -0,0 +1,112 @@
+"""The destinations serialiser's contract: it carries suppression through, and
+never emits a total that closes a gap left by a suppressed category.
+
+The destination categories sum to the cohort, so an aggregate that happens to
+equal the residual names the withheld figure exactly. See
+docs/superpowers/specs/2026-08-28-destination-measures-design.md.
+"""
+
+from backend.data_loader import _destinations_block, _format_cohort_year
+
+
+def _row(group, measure, pupils, status, cohort=180, percentage=None, year=202223):
+    return {
+        "pupil_group": group,
+        "destination_measure": measure,
+        "pupils": pupils,
+        "percentage": percentage,
+        "status": status,
+        "cohort_pupils": cohort,
+        "year": year,
+    }
+
+
+def test_suppressed_category_serialises_as_suppressed_with_null_pupils():
+    rows = [
+        _row("all", "school_sixth_form", 75, "published", percentage=41.7),
+        _row("all", "sixth_form_college", None, "suppressed"),
+    ]
+    block = _destinations_block(rows)
+    cats = {c["category"]: c for c in block["groups"]["all"]["categories"]}
+    assert cats["sixth_form_college"]["status"] == "suppressed"
+    assert cats["sixth_form_college"]["pupils"] is None
+    assert cats["sixth_form_college"]["percentage"] is None
+
+
+def test_published_category_keeps_its_figures():
+    block = _destinations_block([
+        _row("all", "school_sixth_form", 75, "published", percentage=41.7),
+    ])
+    cat = block["groups"]["all"]["categories"][0]
+    assert cat["pupils"] == 75
+    assert cat["percentage"] == 41.7
+    assert cat["status"] == "published"
+
+
+def test_no_closing_total_is_emitted_for_a_partially_suppressed_group():
+    rows = [
+        _row("all", "school_sixth_form", 75, "published", percentage=41.7),
+        _row("all", "sixth_form_college", None, "suppressed"),
+        _row("all", "further_education", 61, "published", percentage=33.9),
+        _row("all", "apprenticeship", 8, "published", percentage=4.4),
+        _row("all", "employment", 6, "published", percentage=3.3),
+        _row("all", "not_sustained", 5, "published", percentage=2.8),
+        _row("all", "not_captured", 4, "published", percentage=2.2),
+    ]
+    block = _destinations_block(rows)
+    group = block["groups"]["all"]
+    published = sum(c["pupils"] for c in group["categories"] if c["pupils"] is not None)
+    residual = group["cohort"] - published
+    for value in group["aggregates"].values():
+        if value is None or value.get("pupils") is None:
+            continue
+        assert value["pupils"] != residual, (
+            "an aggregate equal to the residual identifies the suppressed cell"
+        )
+
+
+def test_aggregates_are_separated_from_categories():
+    rows = [
+        _row("all", "school_sixth_form", 75, "published", percentage=41.7),
+        _row("all", "agg_sustained_all", 171, "published", percentage=95.0),
+    ]
+    block = _destinations_block(rows)
+    group = block["groups"]["all"]
+    assert [c["category"] for c in group["categories"]] == ["school_sixth_form"]
+    assert group["aggregates"]["sustained_all"]["pupils"] == 171
+
+
+def test_only_the_latest_year_is_served():
+    rows = [
+        _row("all", "school_sixth_form", 60, "published", year=202122),
+        _row("all", "school_sixth_form", 75, "published", year=202223),
+    ]
+    block = _destinations_block(rows)
+    assert block["cohort_year"] == "2022/23"
+    assert len(block["groups"]["all"]["categories"]) == 1
+    assert block["groups"]["all"]["categories"][0]["pupils"] == 75
+
+
+def test_all_three_pupil_groups_are_carried():
+    rows = [
+        _row("all", "school_sixth_form", 75, "published"),
+        _row("disadvantaged", "school_sixth_form", 17, "published", cohort=62),
+        _row("other", "school_sixth_form", 58, "published", cohort=118),
+    ]
+    block = _destinations_block(rows)
+    assert set(block["groups"]) == {"all", "disadvantaged", "other"}
+    assert block["groups"]["disadvantaged"]["cohort"] == 62
+
+
+def test_cohort_year_is_reported_so_the_page_can_date_itself():
+    block = _destinations_block([_row("all", "school_sixth_form", 75, "published")])
+    assert block["cohort_year"] == "2022/23"
+
+
+def test_format_cohort_year_handles_the_six_digit_form():
+    assert _format_cohort_year(202223) == "2022/23"
+    assert _format_cohort_year(None) is None
+
+
+def test_empty_rows_yield_none_not_an_empty_shell():
+    assert _destinations_block([]) is None

backend/tests/test_supplementary_batch.py

@@ -132,16 +132,23 @@ def test_one_query_per_table_and_latest_row_per_urn():
         "FactPupilCharacteristics": [],
         "FactDeprivation": [],
         "FactFinance": [],
+        "FactKs4Destinations": [],
+        "FactKs5Destinations": [],
     }
     session = _FakeSession(rows)
     out = get_supplementary_data_batch(session, [1, 2])
 
-    # Exactly one query per table — six total, regardless of two URNs.
+    # Exactly one query per table — eight total, regardless of two URNs.
     assert sorted(session.queries) == [
         "FactAdmissionDistance", "FactAdmissions", "FactDeprivation",
-        "FactFinance", "FactOfstedInspection", "FactPupilCharacteristics",
+        "FactFinance", "FactKs4Destinations", "FactKs5Destinations",
+        "FactOfstedInspection", "FactPupilCharacteristics",
     ]
 
+    # A school with no destination rows gets null, not an empty shell — the
+    # frontend renders the section from the block's presence.
+    assert out[1]["destinations"] is None
+
     # Latest Ofsted kept per URN
     assert out[1]["ofsted"]["overall_effectiveness"] == 2
     assert out[2]["ofsted"]["overall_effectiveness"] == 1