Drag a gas room heater's coherent companions 🟥

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

tests/domain/epc/test_main_heating_system_overlay.py

@@ -583,3 +583,22 @@ def test_gas_room_heater_variants_decode_to_their_sap_codes(
     assert simulation is not None
     assert simulation.heating is not None
     assert simulation.heating.sap_main_heating_code == code
+
+
+def test_gas_room_heater_drags_its_coherent_companions() -> None:
+    # A gas room heater is a room-heater system (category 10) on a single-rate
+    # meter, under the conservative room-heater control (2601). Its natural fuel
+    # is mains gas (26) — an LPG dwelling is refined by a main_fuel override
+    # (the overlay can't see the mains connection) (ADR-0041).
+
+    # Act
+    simulation = main_heating_overlay_for("Gas room heater, condensing fire", 0)
+
+    # Assert
+    assert simulation is not None
+    assert simulation.heating is not None
+    heating = simulation.heating
+    assert heating.main_heating_category == 10
+    assert heating.main_heating_control == 2601
+    assert heating.main_fuel_type == 26
+    assert heating.meter_type == "Single"