Recognise community CHP and boilers as a heat-network system 🟥

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

tests/domain/epc/test_main_heating_system_overlay.py

@@ -524,6 +524,23 @@ def test_community_boilers_drag_heat_network_category_and_a_community_gas_fuel()
     assert heating.main_fuel_type == 20
 
 
+def test_community_chp_and_boilers_decode_to_their_heat_network_code() -> None:
+    # A community scheme with CHP + boilers is SAP Table 4a code 302, still a heat
+    # network (category 6) on community mains gas (20). Real audit data: 10
+    # properties carry this, today mis-bucketed to Gas CPSU (ADR-0041).
+
+    # Act
+    simulation = main_heating_overlay_for("Community heating, CHP and boilers", 0)
+
+    # Assert
+    assert simulation is not None
+    assert simulation.heating is not None
+    heating = simulation.heating
+    assert heating.sap_main_heating_code == 302
+    assert heating.main_heating_category == 6
+    assert heating.main_fuel_type == 20
+
+
 def test_oil_room_heater_decodes_to_the_post_2000_code() -> None:
     # A modern standalone oil room heater is SAP Table 4a code 623 ("Oil room
     # heater, 2000 or later", no boiler). It must decode to 623, not the Gas CPSU