feat(modelling): SecondaryHeatingOverlay clears the lodged secondary (ADR-0028)

The first overlay surface that sets fields to *absent* rather than to a
target state: _fold_secondary_heating clears sap_heating.secondary_heating_type
+ secondary_fuel_type, so the calculator's Table 11 secondary-fraction split
(SAP 10.2 §9a) routes 100% of space heating to the main. On an electric-storage
main RdSAP §A.2.2 re-forces a default secondary, making removal a no-op there —
left to the Optimiser to de-select (ADR-0028 decisions 2-3).

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

domain/modelling/scoring/overlay_applicator.py

@@ -21,6 +21,7 @@ from domain.modelling.simulation import (
     EpcSimulation,
     HeatingOverlay,
     LightingOverlay,
+    SecondaryHeatingOverlay,
     SolarOverlay,
     VentilationOverlay,
     WindowOverlay,
@@ -54,12 +55,29 @@ def apply_simulations(
             _fold_lighting(result, simulation.lighting)
         if simulation.heating is not None:
             _fold_heating(result, simulation.heating)
+        if simulation.secondary_heating is not None:
+            _fold_secondary_heating(result, simulation.secondary_heating)
         if simulation.solar is not None:
             _fold_solar(result, simulation.solar)
 
     return result
 
 
+def _fold_secondary_heating(
+    epc: EpcPropertyData, overlay: SecondaryHeatingOverlay
+) -> None:
+    """Strip the dwelling's lodged secondary heating system (ADR-0028) — the one
+    fold that sets fields to *absent* rather than to a target state. Clears
+    `secondary_heating_type` + `secondary_fuel_type` on `sap_heating`, so the
+    calculator's Table 11 split routes 100% of space heating to the main (or, on
+    an electric-storage main, re-forces the §A.2.2 default — a no-op the
+    Optimiser de-selects)."""
+    if not overlay.remove:
+        return
+    epc.sap_heating.secondary_heating_type = None
+    epc.sap_heating.secondary_fuel_type = None
+
+
 # `HeatingOverlay` fields grouped by the object they target — the deepest fold,
 # spanning the primary `MainHeatingDetail`, `sap_heating`, the top-level
 # `EpcPropertyData`, and `sap_energy_source` (ADR-0024).

domain/modelling/simulation.py

@@ -140,6 +140,24 @@ class HeatingOverlay:
     mains_gas: Optional[bool] = None
 
 
+@dataclass(frozen=True)
+class SecondaryHeatingOverlay:
+    """The change the Secondary Heating Removal Measure makes (ADR-0028): strip
+    the dwelling's lodged secondary heating system so the main serves 100% of
+    space heating. Unlike every other overlay — which writes a *target state*
+    and treats ``None`` as "leave unchanged" — this overlay *clears* the
+    secondary fields (`secondary_heating_type`, `secondary_fuel_type`) to
+    absent. Its presence on an `EpcSimulation` is the signal; `remove` carries
+    the intent explicitly.
+
+    On an electric-storage main RdSAP §A.2.2 forces a default secondary back, so
+    removal is a no-op there — the Optimiser de-selects those (it owns the
+    economics); eligibility still offers them.
+    """
+
+    remove: bool = True
+
+
 @dataclass(frozen=True)
 class SolarOverlay:
     """All-optional partial of the dwelling's PV-bearing energy source — the
@@ -194,4 +212,5 @@ class EpcSimulation:
     ventilation: Optional[VentilationOverlay] = None
     lighting: Optional[LightingOverlay] = None
     heating: Optional[HeatingOverlay] = None
+    secondary_heating: Optional[SecondaryHeatingOverlay] = None
     solar: Optional[SolarOverlay] = None

tests/domain/modelling/test_overlay_applicator.py

@@ -17,6 +17,7 @@ from domain.modelling.simulation import (
     EpcSimulation,
     HeatingOverlay,
     LightingOverlay,
+    SecondaryHeatingOverlay,
     SolarOverlay,
     VentilationOverlay,
     WindowOverlay,
@@ -292,6 +293,38 @@ def test_apply_folds_a_heating_overlay_across_all_five_locations() -> None:
     assert result.sap_energy_source.mains_gas is False
 
 
+def test_secondary_heating_overlay_clears_the_lodged_secondary() -> None:
+    # Arrange — 000490 lodges a secondary system (SAP code 691, electric panel/
+    # convector/radiant heaters). Pin a fuel on it too so we prove the fold
+    # clears BOTH the type and the fuel (ADR-0028).
+    baseline: EpcPropertyData = build_epc()
+    baseline.sap_heating.secondary_fuel_type = 30
+    assert baseline.sap_heating.secondary_heating_type == 691
+
+    # Act — fold a removal overlay.
+    result: EpcPropertyData = apply_simulations(
+        baseline, [EpcSimulation(secondary_heating=SecondaryHeatingOverlay())]
+    )
+
+    # Assert — the secondary is gone from the dwelling handed to the calculator.
+    assert result.sap_heating.secondary_heating_type is None
+    assert result.sap_heating.secondary_fuel_type is None
+
+
+def test_secondary_heating_removal_does_not_mutate_the_baseline() -> None:
+    # Arrange — 000490 lodges secondary SAP code 691.
+    baseline: EpcPropertyData = build_epc()
+    assert baseline.sap_heating.secondary_heating_type == 691
+
+    # Act — fold a removal overlay.
+    _: EpcPropertyData = apply_simulations(
+        baseline, [EpcSimulation(secondary_heating=SecondaryHeatingOverlay())]
+    )
+
+    # Assert — the baseline's secondary is untouched (the fold copies first).
+    assert baseline.sap_heating.secondary_heating_type == 691
+
+
 def test_baseline_heating_is_not_mutated_by_a_heating_overlay() -> None:
     # Arrange — 000490 lodges a mains-gas combi (fuel 26, control 2106, no
     # cylinder, mains_gas True).