Fix #835's claimant guard for child-only benefit units (licensed-build blocker)

The first licensed spine-m build failed at uc_reporter_redraw: 'Every
benefit unit must contain an adult claimant candidate.' A 16-19
qualifying young person heading their own benefit unit is its only
member, so the engine's UC child flag leaves no ~uc_child candidate —
the licensed frame carries 238 all-under-18 benefit units, and PR CI can
never reach this path (secrets-free, no licensed builds).

The fallback now takes the unit's eldest member as its de-facto head,
and the fail-closed check moves to the landing, where a POSITIVE draw on
a child claimant refuses — the award screen keeps child-only units out
of the drawn domain, so a positive landing there is exactly the
corruption the old total guard feared. Regression tests for both halves.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: juaristi22

packages/microcosm-build/src/microcosm/build/uk_runtime/uc_reporter_redraw.py

@@ -276,6 +276,7 @@ def redraw_spi_reported_uc(
         spi=spi,
         claimant_rows=claimant_rows,
         draws=draws,
+        uc_child=uc_child,
     )
     reported_after = _benefit_unit_reporter_amounts(person, benunit)
     if np.any(reported_after[spi & ~screen] != 0.0):
@@ -373,7 +374,13 @@ def _claimant_rows(person: pd.DataFrame, *, uc_child: np.ndarray) -> np.ndarray:
         if not len(candidates):
             candidates = rows[adult[rows]]
         if not len(candidates):
-            raise ValueError("Every benefit unit must contain an adult claimant candidate.")
+            # A 16-19 qualifying young person heading their own benefit unit
+            # is its only member: the engine's UC child flag covers them, so
+            # no ~uc_child candidate exists. The unit's eldest member is its
+            # de-facto head; the fail-closed check moves to the landing,
+            # where a POSITIVE draw on a child claimant refuses (the award
+            # screen keeps such units out of the drawn domain).
+            candidates = rows
         order = np.lexsort((person_id[candidates], -age[candidates]))
         claimant[candidates[order[0]]] = True
     return claimant
@@ -510,6 +517,7 @@ def _land_spi_draws(
     spi: np.ndarray,
     claimant_rows: np.ndarray,
     draws: np.ndarray,
+    uc_child: np.ndarray,
 ) -> None:
     spi_ids = set(benunit.loc[spi, "benunit_id"])
     spi_people = person["person_benunit_id"].isin(spi_ids).to_numpy(dtype=bool)
@@ -519,9 +527,18 @@ def _land_spi_draws(
         draw_by_benunit
     )
     claimant_spi = claimant_rows & spi_people
-    person.loc[claimant_spi, UC_REPORTER_REDRAW_OUTPUT] = claimant_draws.loc[
-        claimant_spi[claimant_rows]
-    ].to_numpy(dtype=float)
+    landed = claimant_draws.loc[claimant_spi[claimant_rows]].to_numpy(dtype=float)
+    child_claimants = uc_child[claimant_spi]
+    if bool((child_claimants & (landed > 0.0)).any()):
+        # The fail-closed half of the child-only-benunit fallback above: the
+        # award screen keeps qualifying-young-person-only units out of the
+        # drawn domain, so a positive amount landing on a child claimant is
+        # the corruption the old total guard feared — refused exactly here.
+        raise ValueError(
+            "A positive UC reporter draw landed on a child claimant; "
+            "the award screen must exclude child-only benefit units."
+        )
+    person.loc[claimant_spi, UC_REPORTER_REDRAW_OUTPUT] = landed
 
 
 def _reporter_transition_receipt(

packages/microcosm-build/tests/test_uk_uc_reporter_redraw.py

@@ -418,3 +418,71 @@ def test_transform_records_checkpoint_metadata() -> None:
     evidence = transform.checkpoint_metadata()["evidence"]
     assert evidence["stage"] == "uc_reporter_redraw"
     assert evidence["seed"] == UC_REPORTER_REDRAW_SEED
+
+
+def test_child_only_benunit_gets_its_eldest_member_as_claimant() -> None:
+    """A qualifying-young-person-only benefit unit must not crash the stage.
+
+    A 16-19 QYP heading their own unit is its only member, so no ~uc_child
+    candidate exists; the unit's eldest member is its de-facto head. The
+    licensed spine carries hundreds of such units, so the old total guard
+    failed every full build.
+    """
+
+    from microcosm.build.uk_runtime.uc_reporter_redraw import _claimant_rows
+
+    person = pd.DataFrame(
+        {
+            "person_id": [1, 2, 3],
+            "person_benunit_id": [10, 10, 20],
+            "age": [40.0, 12.0, 17.0],
+        }
+    )
+    uc_child = np.array([False, True, True])
+
+    claimant = _claimant_rows(person, uc_child=uc_child)
+
+    assert claimant.tolist() == [True, False, True]
+
+
+def test_positive_draw_on_a_child_claimant_refuses_at_the_landing() -> None:
+    """The fail-closed half of the child-only fallback lives at the landing."""
+
+    from microcosm.build.uk_runtime.uc_reporter_redraw import (
+        _claimant_rows,
+        _land_spi_draws,
+    )
+
+    person = pd.DataFrame(
+        {
+            "person_id": [1, 2],
+            "person_benunit_id": [10, 20],
+            "age": [40.0, 17.0],
+            UC_REPORTER_REDRAW_OUTPUT: [0.0, 0.0],
+        }
+    )
+    benunit = pd.DataFrame({"benunit_id": [10, 20]})
+    uc_child = np.array([False, True])
+    claimant = _claimant_rows(person, uc_child=uc_child)
+    spi = np.array([True, True])
+
+    with pytest.raises(ValueError, match="child claimant"):
+        _land_spi_draws(
+            person,
+            benunit,
+            spi=spi,
+            claimant_rows=claimant,
+            draws=np.array([500.0, 500.0]),
+            uc_child=uc_child,
+        )
+
+    zero_for_child = np.array([500.0, 0.0])
+    _land_spi_draws(
+        person,
+        benunit,
+        spi=spi,
+        claimant_rows=claimant,
+        draws=zero_for_child,
+        uc_child=uc_child,
+    )
+    assert person[UC_REPORTER_REDRAW_OUTPUT].tolist() == [500.0, 0.0]