Lock whole-meter off-peak routing and standard-meter no-op 🟩

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

tests/domain/billing/test_energy_breakdown.py

@@ -184,3 +184,50 @@ def test_off_peak_heating_line_carries_its_carrier_and_high_rate_fraction() -> N
         9000.0,
         0.0,
     )
+
+
+def test_off_peak_meter_routes_every_electric_use_to_the_off_peak_carrier() -> None:
+    # Arrange — a gas-heated dwelling that happens to be on an Off-Peak Meter:
+    # the gas heating stays gas, but ALL electric uses (lighting, appliances,
+    # pumps) bill on the off-peak carrier, the "other" uses sharing the
+    # ALL_OTHER_USES fraction and pumps/fans carrying their own.
+    result = _sap_result(
+        main_heating_fuel_kwh_per_yr=8000.0,
+        main_heating_fuel_code=1,  # mains gas — single-rate, unaffected
+        lighting_kwh_per_yr=400.0,
+        appliances_kwh_per_yr=1900.0,
+        pumps_fans_kwh_per_yr=200.0,
+        is_off_peak_meter=True,
+        other_electricity_high_rate_fraction=0.90,
+        pumps_fans_high_rate_fraction=0.71,
+    )
+
+    # Act
+    breakdown = EnergyBreakdown.from_sap_result(result)
+
+    # Assert
+    by_section = {line.section: line for line in breakdown.lines}
+    assert by_section[BillSection.HEATING].fuel is Fuel.MAINS_GAS
+    assert by_section[BillSection.HEATING].high_rate_fraction is None
+    assert by_section[BillSection.LIGHTING].fuel is Fuel.ELECTRICITY_OFF_PEAK
+    assert by_section[BillSection.LIGHTING].high_rate_fraction == 0.90
+    assert by_section[BillSection.APPLIANCES].fuel is Fuel.ELECTRICITY_OFF_PEAK
+    assert by_section[BillSection.APPLIANCES].high_rate_fraction == 0.90
+    assert by_section[BillSection.PUMPS_FANS].high_rate_fraction == 0.71
+
+
+def test_standard_meter_keeps_electric_uses_on_flat_rate_electricity() -> None:
+    # Arrange — electric heating on a standard meter: no day/night split anywhere.
+    result = _sap_result(
+        main_heating_fuel_kwh_per_yr=6000.0,
+        main_heating_fuel_code=30,  # standard-tariff electricity
+        lighting_kwh_per_yr=400.0,
+        is_off_peak_meter=False,
+    )
+
+    # Act
+    breakdown = EnergyBreakdown.from_sap_result(result)
+
+    # Assert — every line is flat-rate ELECTRICITY with no high-rate fraction.
+    assert all(line.fuel is Fuel.ELECTRICITY for line in breakdown.lines)
+    assert all(line.high_rate_fraction is None for line in breakdown.lines)