Resolve a landlord from-main-gas water-heating override to its codes 🟥

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

domain/epc/property_overlays/water_heating_overlay.py

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+"""Map a Landlord-Override water-heating value to a heating Simulation Overlay.
+
+A water-heating value is one canonical "<system>, <fuel>" description ("From main
+system, mains gas", "Electric immersion, electricity"). The calculator reads the
+hot-water arrangement from `sap_heating.water_heating_code` (the SAP Table 4a
+system code) and `water_heating_fuel`, so the overlay decomposes the value into
+those two int codes and emits a whole-dwelling `HeatingOverlay` (water heating is
+not per-building-part, so `building_part` is ignored). It composes field-wise with
+the main_fuel / main_heating overlays. Unresolvable values produce no overlay.
+"""
+
+from __future__ import annotations
+
+from typing import Optional
+
+from domain.modelling.simulation import EpcSimulation
+
+
+def water_heating_overlay_for(
+    water_heating_value: str, building_part: int
+) -> Optional[EpcSimulation]:
+    raise NotImplementedError

tests/domain/epc/test_water_heating_overlay.py

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+"""The Landlord-Override water-heating → heating Simulation Overlay mapping.
+
+A water-heating value resolves to the SAP `water_heating_code` (system) and
+`water_heating_fuel` the calculator reads; the overlay is whole-dwelling.
+"""
+
+from __future__ import annotations
+
+from domain.epc.property_overlays.water_heating_overlay import (
+    water_heating_overlay_for,
+)
+
+
+def test_from_main_system_mains_gas_overlays_water_heating() -> None:
+    # Act
+    simulation = water_heating_overlay_for("From main system, mains gas", 0)
+
+    # Assert — "from main system" is water_heating_code 901, mains gas fuel 26.
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.water_heating_code == 901
+    assert simulation.heating.water_heating_fuel == 26