Resolve a landlord gas-combi heating override to its SAP code 🟥

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

domain/epc/property_overlays/main_heating_system_overlay.py

@@ -0,0 +1,28 @@
+"""Map a Landlord-Override main-heating-system value to a heating Simulation Overlay.
+
+A main-heating-system value is one canonical system archetype ("Gas boiler,
+combi", "Electric storage heaters, fan"). The calculator reads the primary
+system's `sap_main_heating_code` (SAP Table 4a/4b), so the overlay maps the
+archetype to a representative code and emits a whole-dwelling `HeatingOverlay`
+targeting `main_heating_details[0]` (`building_part` is ignored). It composes
+field-wise with the main_fuel / water_heating overlays.
+
+The SEDBUK A-G efficiency band the Hyde "Heating" column carries is NOT honoured
+yet (no efficiency slot on the overlay/MainHeatingDetail) -- archetypes map to
+their modern/condensing Table 4b code, so an old low-rated boiler is currently
+modelled at the condensing efficiency. Heat pumps and community heating (which
+resolve via main_heating_index_number / community codes, not a Table 4b code)
+are left UNKNOWN until modelled. Unresolvable values produce no overlay.
+"""
+
+from __future__ import annotations
+
+from typing import Optional
+
+from domain.modelling.simulation import EpcSimulation
+
+
+def main_heating_overlay_for(
+    main_heating_value: str, building_part: int
+) -> Optional[EpcSimulation]:
+    raise NotImplementedError

tests/domain/epc/test_main_heating_system_overlay.py

@@ -0,0 +1,21 @@
+"""The Landlord-Override main-heating-system → heating Simulation Overlay mapping.
+
+A main-heating-system value resolves to the SAP `sap_main_heating_code` the
+calculator reads from the primary system; the overlay is whole-dwelling.
+"""
+
+from __future__ import annotations
+
+from domain.epc.property_overlays.main_heating_system_overlay import (
+    main_heating_overlay_for,
+)
+
+
+def test_gas_combi_overlays_the_primary_heating_code() -> None:
+    # Act
+    simulation = main_heating_overlay_for("Gas boiler, combi", 0)
+
+    # Assert — condensing combi is SAP Table 4b code 104.
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.sap_main_heating_code == 104