Drag heat-network category and a community gas fuel for community boilers 🟥

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

tests/domain/epc/test_main_heating_system_overlay.py

@@ -504,3 +504,21 @@ def test_community_boilers_decode_to_the_heat_network_boiler_code() -> None:
     assert simulation is not None
     assert simulation.heating is not None
     assert simulation.heating.sap_main_heating_code == 301
+
+
+def test_community_boilers_drag_heat_network_category_and_a_community_gas_fuel() -> None:
+    # A community boiler scheme is SAP main_heating_category 6 (heat network), so
+    # the calculator treats it as a heat network (cert_to_inputs `_is_heat_network`
+    # checks code OR category 6). Its natural fuel is mains gas (community) — RdSAP
+    # main_fuel code 20, the dominant fuel real community-boiler certs lodge. A
+    # specific main_fuel override (e.g. biomass community) still wins (ADR-0041).
+
+    # Act
+    simulation = main_heating_overlay_for("Community heating, boilers", 0)
+
+    # Assert
+    assert simulation is not None
+    assert simulation.heating is not None
+    heating = simulation.heating
+    assert heating.main_heating_category == 6
+    assert heating.main_fuel_type == 20