Remap the EPC primary fuel from a landlord fuel override 🟩

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

tests/domain/epc/test_main_fuel_overlay.py

@@ -9,6 +9,10 @@ from __future__ import annotations
 import pytest
 
 from domain.epc.property_overlays.main_fuel_overlay import fuel_overlay_for
+from domain.modelling.scoring.overlay_applicator import apply_simulations
+from tests.domain.sap10_calculator.worksheet._elmhurst_worksheet_000490 import (
+    build_epc,
+)
 
 
 def test_mains_gas_overlays_the_primary_fuel() -> None:
@@ -59,3 +63,16 @@ def test_unresolvable_fuel_produces_no_overlay(main_fuel_value: str) -> None:
 
     # Assert
     assert simulation is None
+
+
+def test_fuel_override_remaps_the_primary_systems_fuel_on_the_epc() -> None:
+    # Arrange — a landlord correction that the dwelling runs on electricity.
+    baseline = build_epc()
+    overlay = fuel_overlay_for("electricity", 0)
+    assert overlay is not None
+
+    # Act
+    result = apply_simulations(baseline, [overlay])
+
+    # Assert — the calculator reads the primary fuel from main_heating_details[0].
+    assert result.sap_heating.main_heating_details[0].main_fuel_type == 29