P5.5: SapResult.intermediate exposes useful_space_heating_kwh_per_yr

§9 / Table 9c step 10 output keyed by worksheet name on `intermediate`.
Mirrors the top-level `space_heating_kwh_per_yr` field so spec sweep
slices refer to the worksheet name regardless of field renames.

135 SAP tests pass.

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

@@ -351,6 +351,7 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         "time_constant_h": tau_h,
         "internal_gains_annual_avg_w": sum(e.internal_gains_w for e in monthly) / 12.0,
         "mean_internal_temp_annual_avg_c": sum(e.internal_temp_c for e in monthly) / 12.0,
+        "useful_space_heating_kwh_per_yr": space_heating_kwh,
     }
 
     return SapResult(

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

@@ -212,6 +212,23 @@ def test_calculate_exposes_hlc_hlp_and_annual_averages() -> None:
     )
 
 
+def test_calculate_exposes_useful_space_heating_kwh() -> None:
+    # Arrange — P5 trace mode: useful space heating kWh/yr (§9 / Table 9c
+    # step 10) surfaces on `intermediate` keyed by worksheet name. Mirrors
+    # `space_heating_kwh_per_yr` on the top-level result so spec sweep
+    # slices can refer to the worksheet name regardless of `SapResult`
+    # field renames.
+    inputs = _baseline_inputs()
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert
+    assert result.intermediate["useful_space_heating_kwh_per_yr"] == pytest.approx(
+        result.space_heating_kwh_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.