P5.3: SapResult.intermediate exposes ventilation group

infiltration_ach (the cert-derived input) and infiltration_w_per_k
(the derived HLC_V = ACH × volume × 0.33 from SAP 10.2 §4.1) populated
on `intermediate`. Diagnostic surface for the §4 / Table 4g sweep.

133 SAP tests pass.

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

@@ -344,6 +344,8 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         "windows_w_per_k": ht.windows_w_per_k,
         "doors_w_per_k": ht.doors_w_per_k,
         "thermal_bridging_w_per_k": ht.thermal_bridging_w_per_k,
+        "infiltration_ach": inputs.infiltration_ach,
+        "infiltration_w_per_k": hlc_v,
     }
 
     return SapResult(

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

@@ -159,6 +159,24 @@ def test_calculate_exposes_heat_transmission_intermediates() -> None:
     assert result.intermediate["thermal_bridging_w_per_k"] == ht.thermal_bridging_w_per_k
 
 
+def test_calculate_exposes_ventilation_intermediates() -> None:
+    # Arrange — P5 trace mode: infiltration ach (the cert-derived input) and
+    # the derived ventilation heat-loss W/K must surface so §4 / Table 4g
+    # sweep slices can diff per-cert against the spec formula
+    # HLC_V = ACH × volume × 0.33 (SAP 10.2 §4.1).
+    inputs = _baseline_inputs()
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert
+    assert result.intermediate["infiltration_ach"] == inputs.infiltration_ach
+    expected_hlc_v = inputs.infiltration_ach * inputs.dimensions.volume_m3 * 0.33
+    assert result.intermediate["infiltration_w_per_k"] == pytest.approx(
+        expected_hlc_v, 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.