Audit: pin u_wall description cascade against RdSAP10 Table 6 (England) for golden cert cohort

Worksheet fixtures lodge walls=[] so the description-driven branches of u_wall — the codepath real API certs trigger — were never validated at cascade level. New parametrised test pins each (description, age) pair seen in the 8 golden certs against the Table 6 value the spec mandates. All 7 clean cases match spec: the description cascade is correct where Table 6 gives a direct value. Cases routing through §5.7 / §5.8 formulas are excluded pending separate pinning.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py

@@ -16,6 +16,8 @@ defaults -> country defaults -> final mid-range value so that callers can
 treat the envelope as if RdSAP had assigned an as-built default.
 """
 
+from typing import Optional
+
 import pytest
 
 from domain.ml.rdsap_uvalues import (
@@ -1166,3 +1168,65 @@ def test_u_rr_stud_wall_thickness_125mm_takes_nearest_tabulated_row_below() -> N
 
     # Assert
     assert result == pytest.approx(0.31, abs=0.001)
+
+
+# ----- Description-cascade cohort pins (Walls) -----
+#
+# The Elmhurst worksheet fixtures lodge `walls=[]` and so cannot exercise
+# the description-driven branches of `u_wall`. The 8 golden API certs DO
+# carry `walls[0].description` strings that route through `_described_as_
+# insulated`, `_wall_type_from_description`, and the Table 6 footnote
+# 50 mm-bucket override. These tests pin every (description, age) pair
+# seen in that cohort against the RdSAP10 Table 6 (England) value the
+# spec mandates — closing the cascade-coverage gap identified during the
+# 2026-05-24 audit (description cascade was 100% spec-correct on clean
+# Table 6 rows; this test locks that in for regression).
+#
+# Cases routing through §5.7 (solid brick from wall thickness) or §5.8
+# (stone/brick with insulation, ages A–D — formula not table) are
+# intentionally excluded — they need separate pinning when those
+# formulas land.
+_TABLE_6_ENG_WALL_COHORT_PINS: tuple[tuple[str, str, Optional[int], Optional[int], Optional[int], bool, float], ...] = (
+    # (description, age_band, wall_construction, wall_insulation_type,
+    #  insulation_thickness_mm, insulation_present, expected_u_w_per_m2k)
+    # `insulation_present` mirrors the heat_transmission cascade: type != 4 (NONE)
+    # OR description asserts insulation per _described_as_insulated.
+    ("Sandstone, as built, insulated (assumed)",       "J", 2, 4, 0, True,  0.25),   # cert 0240 (50 mm bucket per footnote)
+    ("Cavity wall, filled cavity",                     "C", 4, 2, 0, True,  0.7),    # cert 8135 bp0
+    ("Cavity wall, filled cavity",                     "D", 4, 2, 0, True,  0.7),    # cert 0300, 7536 bp0
+    ("Cavity wall, filled cavity",                     "F", 4, 2, 0, True,  0.40),   # cert 7536 bp2
+    ("Cavity wall, filled cavity",                     "G", 4, 2, 0, True,  0.35),   # cert 8135 bp1
+    ("Cavity wall, filled cavity",                     "L", 4, 4, 0, False, 0.28),   # cert 7536 bp1 (assumed-as-built †)
+    ("Cavity wall, as built, no insulation (assumed)", "D", 4, 4, 0, False, 1.5),    # cert 0390-2954, 9390
+)
+
+
+@pytest.mark.parametrize(
+    "description, age_band, construction, insulation_type, thickness_mm, insulation_present, expected_u",
+    _TABLE_6_ENG_WALL_COHORT_PINS,
+)
+def test_u_wall_matches_table6_for_every_cohort_description_age_pair(
+    description: str,
+    age_band: str,
+    construction: Optional[int],
+    insulation_type: Optional[int],
+    thickness_mm: Optional[int],
+    insulation_present: bool,
+    expected_u: float,
+) -> None:
+    # Arrange — inputs replicate what `heat_transmission_from_cert` feeds
+    # `u_wall` for the corresponding building part in the golden cert cohort.
+
+    # Act
+    u = u_wall(
+        country=Country.ENG,
+        age_band=age_band,
+        construction=construction,
+        insulation_thickness_mm=thickness_mm,
+        insulation_present=insulation_present,
+        description=description,
+        wall_insulation_type=insulation_type,
+    )
+
+    # Assert
+    assert abs(u - expected_u) < 1e-4