Resolve a landlord mains-gas override to the primary fuel code 🟥

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

domain/epc/property_overlays/main_fuel_overlay.py

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+"""Map a Landlord-Override main-fuel value to a heating Simulation Overlay.
+
+A main-fuel value is one canonical gov-EPC `main_fuel` description ("mains gas",
+"electricity", …). The calculator reads the dwelling's primary fuel from
+`main_heating_details[0].main_fuel_type` as the RdSAP **int code**, so the
+overlay decomposes the value into that code and emits a whole-dwelling
+`HeatingOverlay` (fuel is not a per-building-part attribute, so `building_part`
+is ignored). Codes follow the modern RdSAP-20/21 `(not community)` family the
+gov-EPC API baseline uses. Unresolvable values produce no overlay.
+"""
+
+from __future__ import annotations
+
+from typing import Optional
+
+from domain.modelling.simulation import EpcSimulation
+
+
+def fuel_overlay_for(
+    main_fuel_value: str, building_part: int
+) -> Optional[EpcSimulation]:
+    raise NotImplementedError

tests/domain/epc/test_main_fuel_overlay.py

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+"""The Landlord-Override main-fuel → heating Simulation Overlay mapping.
+
+A main-fuel value resolves to the RdSAP `main_fuel_type` int code the calculator
+reads from the dwelling's primary heating system; the overlay is whole-dwelling.
+"""
+
+from __future__ import annotations
+
+from domain.epc.property_overlays.main_fuel_overlay import fuel_overlay_for
+
+
+def test_mains_gas_overlays_the_primary_fuel() -> None:
+    # Act
+    simulation = fuel_overlay_for("mains gas", 0)
+
+    # Assert — mains gas (not community) is RdSAP main_fuel code 26.
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.main_fuel_type == 26