Remap the primary heating system and compose the heating override trio 🟩

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

tests/domain/epc/test_main_heating_system_overlay.py

@@ -8,9 +8,17 @@ from __future__ import annotations
 
 import pytest
 
+from domain.epc.property_overlays.main_fuel_overlay import fuel_overlay_for
 from domain.epc.property_overlays.main_heating_system_overlay import (
     main_heating_overlay_for,
 )
+from domain.epc.property_overlays.water_heating_overlay import (
+    water_heating_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_gas_combi_overlays_the_primary_heating_code() -> None:
@@ -59,3 +67,39 @@ def test_unresolvable_or_unmodelled_heating_produces_no_overlay(
 
     # Assert
     assert simulation is None
+
+
+def test_main_heating_override_remaps_the_primary_system_code() -> None:
+    # Arrange
+    baseline = build_epc()
+    overlay = main_heating_overlay_for("Gas boiler, regular", 0)
+    assert overlay is not None
+
+    # Act
+    result = apply_simulations(baseline, [overlay])
+
+    # Assert — the calculator reads the code off main_heating_details[0].
+    assert result.sap_heating.main_heating_details[0].sap_main_heating_code == 102
+
+
+def test_the_three_heating_overrides_compose_without_conflict() -> None:
+    # Arrange — main_fuel, water_heating and main_heating_system all fold onto one
+    # HeatingOverlay surface but set DISJOINT fields, so they compose (the
+    # field-disjoint design that makes precedence moot for these three).
+    baseline = build_epc()
+    overlays = [
+        fuel_overlay_for("electricity", 0),
+        water_heating_overlay_for("Electric immersion, electricity", 0),
+        main_heating_overlay_for("Electric storage heaters, fan", 0),
+    ]
+    assert all(o is not None for o in overlays)
+
+    # Act
+    result = apply_simulations(baseline, [o for o in overlays if o is not None])
+
+    # Assert — each override landed on its own field.
+    main = result.sap_heating.main_heating_details[0]
+    assert main.main_fuel_type == 29
+    assert main.sap_main_heating_code == 404
+    assert result.sap_heating.water_heating_code == 903
+    assert result.sap_heating.water_heating_fuel == 29