Preserve an existing more-off-peak cert meter under a heating overlay 🟥

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

tests/domain/epc/test_main_heating_system_overlay.py

@@ -329,6 +329,35 @@ def test_the_three_heating_overrides_compose_without_conflict() -> None:
     assert result.sap_heating.water_heating_fuel == 29
 
 
+def test_room_heaters_preserve_an_existing_more_off_peak_cert_meter() -> None:
+    # The overlay's assumed Dual (7-hour E7) meter is a coherent *default* for a
+    # single/unknown-meter dwelling — it must NOT downgrade a cert that already
+    # lodges a more-off-peak meter (here a 24-hour all-low tariff, code "4").
+    # Clobbering it to E7 would bill the heating on a high/low split it doesn't
+    # have, under-rating the dwelling.
+    baseline = build_epc()
+    baseline.sap_energy_source.meter_type = "4"  # 24-hour tariff
+    overlay = main_heating_overlay_for("Electric room heaters", 0)
+    assert overlay is not None
+
+    result = apply_simulations(baseline, [overlay])
+
+    assert result.sap_energy_source.meter_type == "4"
+
+
+def test_room_heaters_set_dual_when_the_cert_meter_is_single() -> None:
+    # The flip side: a single-rate dwelling DOES get the assumed Dual meter —
+    # off-peak heating can't be billed on a single-rate meter (ADR-0035 drag).
+    baseline = build_epc()
+    baseline.sap_energy_source.meter_type = "Single"
+    overlay = main_heating_overlay_for("Electric room heaters", 0)
+    assert overlay is not None
+
+    result = apply_simulations(baseline, [overlay])
+
+    assert result.sap_energy_source.meter_type == "Dual"
+
+
 def test_electric_room_heaters_member_decodes_to_the_room_heater_code() -> None:
     # Arrange — the canonical landlord archetype for direct-acting room heaters
     member = MainHeatingSystemType.ELECTRIC_ROOM_HEATERS