Recommend loft insulation for a loft below the building-regs depth 🟥

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

tests/domain/modelling/test_roof_recommendation.py

@@ -216,6 +216,32 @@ def test_unlodged_roof_type_with_nd_thickness_yields_no_recommendation() -> None
     assert recommendation is None
 
 
+def test_loft_below_building_regs_depth_yields_loft_insulation() -> None:
+    # Arrange — property 724702's shape (ADR-0063): an unlodged roof type with a
+    # lodged 50 mm loft, below the 270 mm building-regs compliance gate. A
+    # measured depth is a positive statement of a real loft (unlike a sentinel on
+    # an unlodged type, which keeps deferring), so the below-regs top-up fires and
+    # reaches the 300 mm install depth.
+    baseline: EpcPropertyData = build_epc()
+    main: SapBuildingPart = _part(baseline, BuildingPartIdentifier.MAIN)
+    main.roof_construction_type = None
+    main.roof_insulation_thickness = "50mm"
+
+    # Act
+    recommendation: Recommendation | None = recommend_roof_insulation(
+        baseline, _StubProducts()
+    )
+
+    # Assert
+    assert recommendation is not None
+    option = recommendation.options[0]
+    assert option.measure_type == "loft_insulation"
+    simulated: EpcPropertyData = apply_simulations(baseline, [option.overlay])
+    assert (
+        _part(simulated, BuildingPartIdentifier.MAIN).roof_insulation_thickness == 300
+    )
+
+
 def test_loft_option_carries_cost_from_roof_area_and_product() -> None:
     # Arrange
     baseline: EpcPropertyData = build_epc()  # MAIN roof area 14.85 m^2