P5.8: SapResult.intermediate exposes CO2 chain

Adds delivered_fuel_kwh_per_yr (sum of all five end-use kWh) and
co2_factor_kg_per_kwh (mirrors the SAP10 input). Together with the
top-level co2_kg_per_yr they make the §15 equation traceable:
co2 = delivered_fuel × factor.

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

@@ -371,6 +371,8 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         "lighting_cost_gbp": lighting_cost,
         "ecf": ecf,
         "deflator": ENERGY_COST_DEFLATOR,
+        "delivered_fuel_kwh_per_yr": delivered_fuel_kwh,
+        "co2_factor_kg_per_kwh": inputs.co2_factor_kg_per_kwh,
     }
 
     return SapResult(

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

@@ -278,6 +278,36 @@ def test_calculate_exposes_ecf_and_deflator() -> None:
     assert result.intermediate["deflator"] == pytest.approx(0.36, rel=1e-12)
 
 
+def test_calculate_exposes_co2_chain() -> None:
+    # Arrange — P5 trace mode: CO2 = delivered_fuel × co2_factor. Both
+    # inputs surface on `intermediate` so the top-level co2_kg_per_yr is
+    # auditable. Delivered fuel is the sum of every end-use kWh; the
+    # factor mirrors the SAP10 inputs.co2_factor_kg_per_kwh.
+    inputs = _baseline_inputs()
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert
+    expected_delivered = (
+        result.main_heating_fuel_kwh_per_yr
+        + result.secondary_heating_fuel_kwh_per_yr
+        + result.hot_water_kwh_per_yr
+        + result.pumps_fans_kwh_per_yr
+        + result.lighting_kwh_per_yr
+    )
+    assert result.intermediate["delivered_fuel_kwh_per_yr"] == pytest.approx(
+        expected_delivered, rel=1e-9
+    )
+    assert result.intermediate["co2_factor_kg_per_kwh"] == pytest.approx(
+        inputs.co2_factor_kg_per_kwh, rel=1e-12
+    )
+    assert (
+        result.intermediate["delivered_fuel_kwh_per_yr"]
+        * result.intermediate["co2_factor_kg_per_kwh"]
+    ) == pytest.approx(result.co2_kg_per_yr, rel=1e-9)
+
+
 def test_higher_main_heating_efficiency_reduces_fuel_use() -> None:
     # Arrange — Direction check: doubling the boiler efficiency must halve
     # the main-heating fuel kWh, holding everything else constant.