feat(modelling): price the cavity Option from area x Product — closes #1155

recommend_cavity_wall now takes a ProductRepository and prices the Measure
Option: Cost(total = gross_heat_loss_wall_area(MAIN) x product.unit_cost_per_m2,
contingency_rate = product.contingency_rate). Detection is unchanged and runs
before pricing, so ineligible walls still return None without a catalogue hit.

Completes #1155 — the cavity-wall Recommendation Generator now detects an
uninsulated main cavity wall and emits a priced Option carrying the filled-
cavity overlay. Four behaviour tests (detection x3 + fully-loaded cost).
pyright strict clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

domain/modelling/wall_recommendation.py

@@ -11,8 +11,12 @@ from datatypes.epc.domain.epc_property_data import (
     BuildingPartIdentifier,
     EpcPropertyData,
 )
-from domain.modelling.recommendation import MeasureOption, Recommendation
+from domain.building_geometry import gross_heat_loss_wall_area
+from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
 from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
+from repositories.product.product_repository import ProductRepository
+
+_CAVITY_MEASURE_TYPE = "cavity_wall_insulation"
 
 # RdSAP 10 Table 5 wall_construction: 4 = "Cavity". Table 6
 # wall_insulation_type: 4 = "as-built / assumed" (uninsulated), 2 = "Filled
@@ -23,9 +27,12 @@ _WALL_UNINSULATED = 4
 _FILLED_CAVITY = 2
 
 
-def recommend_cavity_wall(epc: EpcPropertyData) -> Optional[Recommendation]:
+def recommend_cavity_wall(
+    epc: EpcPropertyData, products: ProductRepository
+) -> Optional[Recommendation]:
     """Return a cavity-fill Recommendation for the main wall when it is an
-    uninsulated cavity wall, else None."""
+    uninsulated cavity wall, else None. The Option's cost is the heat-loss wall
+    area priced at the Product's fully-loaded unit cost, with its contingency."""
     main = next(
         part
         for part in epc.sap_building_parts
@@ -38,8 +45,15 @@ def recommend_cavity_wall(epc: EpcPropertyData) -> Optional[Recommendation]:
     ):
         return None
 
+    product = products.get(_CAVITY_MEASURE_TYPE)
+    wall_area: float = gross_heat_loss_wall_area(epc, BuildingPartIdentifier.MAIN)
+    cost = Cost(
+        total=wall_area * product.unit_cost_per_m2,
+        contingency_rate=product.contingency_rate,
+    )
+
     option = MeasureOption(
-        measure_type="cavity_wall_insulation",
+        measure_type=_CAVITY_MEASURE_TYPE,
         description="Cavity wall insulation (fill the existing cavity)",
         overlay=EpcSimulation(
             building_parts={
@@ -48,5 +62,6 @@ def recommend_cavity_wall(epc: EpcPropertyData) -> Optional[Recommendation]:
                 )
             }
         ),
+        cost=cost,
     )
     return Recommendation(surface="Main wall", options=(option,))

tests/domain/modelling/test_wall_recommendation.py

@@ -9,13 +9,24 @@ from datatypes.epc.domain.epc_property_data import (
     SapBuildingPart,
 )
 from domain.modelling.overlay_applicator import apply_simulations
+from domain.modelling.product import Product
 from domain.modelling.recommendation import Recommendation
 from domain.modelling.wall_recommendation import recommend_cavity_wall
+from repositories.product.product_repository import ProductRepository
 from tests.domain.sap10_calculator.worksheet._elmhurst_worksheet_000490 import (
     build_epc,
 )
 
 
+class _StubProducts(ProductRepository):
+    """In-memory ProductRepository returning a fixed cavity Product."""
+
+    def get(self, measure_type: str) -> Product:
+        return Product(
+            measure_type=measure_type, unit_cost_per_m2=18.5, contingency_rate=0.10
+        )
+
+
 def _part(epc: EpcPropertyData, identifier: BuildingPartIdentifier) -> SapBuildingPart:
     return next(p for p in epc.sap_building_parts if p.identifier is identifier)
 
@@ -25,7 +36,7 @@ def test_uninsulated_main_cavity_wall_yields_a_cavity_fill_recommendation() -> N
     baseline: EpcPropertyData = build_epc()  # MAIN: cavity (4), uninsulated (4)
 
     # Act
-    recommendation: Recommendation | None = recommend_cavity_wall(baseline)
+    recommendation: Recommendation | None = recommend_cavity_wall(baseline, _StubProducts())
 
     # Assert
     assert recommendation is not None
@@ -43,12 +54,28 @@ def test_already_insulated_main_wall_yields_no_recommendation() -> None:
     _part(baseline, BuildingPartIdentifier.MAIN).wall_insulation_type = 2  # filled
 
     # Act
-    recommendation: Recommendation | None = recommend_cavity_wall(baseline)
+    recommendation: Recommendation | None = recommend_cavity_wall(baseline, _StubProducts())
 
     # Assert
     assert recommendation is None
 
 
+def test_cavity_option_carries_fully_loaded_cost_from_area_and_product() -> None:
+    # Arrange
+    baseline: EpcPropertyData = build_epc()  # MAIN gross heat-loss area 45.93 m^2
+    products = _StubProducts()  # cavity 18.5 GBP/m^2, contingency 0.10
+
+    # Act
+    recommendation: Recommendation | None = recommend_cavity_wall(baseline, products)
+
+    # Assert
+    assert recommendation is not None
+    cost = recommendation.options[0].cost
+    assert cost is not None
+    assert abs(cost.total - 45.93 * 18.5) <= 0.01
+    assert abs(cost.contingency_rate - 0.10) <= 1e-9
+
+
 def test_non_cavity_main_wall_yields_no_recommendation() -> None:
     # Arrange
     baseline: EpcPropertyData = build_epc()
@@ -56,7 +83,7 @@ def test_non_cavity_main_wall_yields_no_recommendation() -> None:
     main.wall_construction = 2  # solid (not cavity); still uninsulated
 
     # Act
-    recommendation: Recommendation | None = recommend_cavity_wall(baseline)
+    recommendation: Recommendation | None = recommend_cavity_wall(baseline, _StubProducts())
 
     # Assert
     assert recommendation is None