P5.11: SapResult.intermediate exposes PV export credit

Final P5 slice. PV credit was the missing term linking the per-end-use
fuel costs (P5.6) to the top-level total_fuel_cost_gbp: total =
max(0, sum(per-end-use) − pv_credit). With this key, every step of
the §13 cost chain — per-fuel cost → PV credit → total → ECF →
rating — is auditable from `intermediate`. P5 trace exposure is
complete.

packages/domain/src/domain/sap/calculator.py

@@ -375,6 +375,7 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         "hot_water_cost_gbp": hot_water_cost,
         "pumps_fans_cost_gbp": pumps_fans_cost,
         "lighting_cost_gbp": lighting_cost,
+        "pv_export_credit_gbp": pv_credit,
         "ecf": ecf,
         "deflator": ENERGY_COST_DEFLATOR,
         "delivered_fuel_kwh_per_yr": delivered_fuel_kwh,

packages/domain/src/domain/sap/tests/test_calculator.py

@@ -348,6 +348,39 @@ def test_calculate_exposes_primary_energy_breakdown() -> None:
     assert result.primary_energy_kwh_per_yr == pytest.approx(expected_total, rel=1e-9)
 
 
+def test_calculate_exposes_pv_export_credit() -> None:
+    # Arrange — P5 trace mode: total_fuel_cost_gbp = sum(per-end-use
+    # costs) − pv_export_credit, floored at 0. The PV credit is the only
+    # missing term linking the P5.6 per-end-use cost breakdown to the
+    # top-level total. Set non-zero PV values so the credit is meaningful.
+    inputs = replace(
+        _baseline_inputs(),
+        pv_generation_kwh_per_yr=1000.0,
+        pv_export_credit_gbp_per_kwh=0.05,
+    )
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert
+    expected_credit = (
+        inputs.pv_generation_kwh_per_yr * inputs.pv_export_credit_gbp_per_kwh
+    )
+    assert result.intermediate["pv_export_credit_gbp"] == pytest.approx(
+        expected_credit, rel=1e-12
+    )
+    gross_cost = (
+        result.intermediate["main_heating_cost_gbp"]
+        + result.intermediate["secondary_heating_cost_gbp"]
+        + result.intermediate["hot_water_cost_gbp"]
+        + result.intermediate["pumps_fans_cost_gbp"]
+        + result.intermediate["lighting_cost_gbp"]
+    )
+    assert max(0.0, gross_cost - expected_credit) == pytest.approx(
+        result.total_fuel_cost_gbp, rel=1e-9
+    )
+
+
 def test_calculate_exposes_rating_equation_spec_constants() -> None:
     # Arrange — P5 trace mode: the §13 ECF denominator carries a 45 m²
     # floor-area offset (Table 12) and the SAP rating splits between a