Drag an oil 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

@@ -536,3 +536,22 @@ def test_oil_room_heater_decodes_to_the_post_2000_code() -> None:
     assert simulation is not None
     assert simulation.heating is not None
     assert simulation.heating.sap_main_heating_code == 623
+
+
+def test_oil_room_heater_drags_its_coherent_room_heater_companions() -> None:
+    # An oil room heater is a room-heater system (category 10) on a single-rate
+    # meter, under the conservative room-heater control (2601), with its natural
+    # fuel heating oil (RdSAP main_fuel 28). The overlay drags these so a
+    # system-only override is self-coherent (ADR-0035 / ADR-0041).
+
+    # Act
+    simulation = main_heating_overlay_for("Oil room heater, 2000 or later", 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 == 28
+    assert heating.meter_type == "Single"