test(calculator): RED — overlay (winter,summer) slot wins over Table 4b code

The override-supplied seasonal efficiency must take precedence in both the §206
winter path (main_heating_efficiency) and the Appendix D Eq D1 summer water path,
the same precedence slot the PCDB winter_efficiency_pct occupies (ADR-0068).

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

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -2885,6 +2885,65 @@ def test_living_area_rounds_half_up_at_2_dp_decimal_boundary_per_rdsap_15() -> N
     assert abs(fla_off_boundary - (14.06 / 46.87)) <= 1e-12
 
 
+def _epc_with_override_boiler(
+    base: object, seasonal_efficiency_override_pct: Optional[tuple[float, float]]
+):
+    """A copy of the typical semi-detached cert whose primary gas boiler (Table 4b
+    code 102) carries a Landlord-Override `(winter, summer)` efficiency slot — the
+    fifth Heating Companion (ADR-0068)."""
+    import dataclasses
+
+    typical = _typical_semi_detached_epc()
+    detail = dataclasses.replace(
+        _gas_boiler_detail(sap_main_heating_code=102),
+        seasonal_efficiency_override_pct=seasonal_efficiency_override_pct,
+    )
+    return make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        region_code="1",
+        sap_building_parts=typical.sap_building_parts,
+        sap_windows=typical.sap_windows,
+        sap_heating=make_sap_heating(main_heating_details=[detail]),
+    )
+
+
+def test_seasonal_efficiency_override_wins_over_the_table_4b_code_default() -> None:
+    # A gas boiler override forces the SEDBUK-band efficiency ahead of the Table
+    # 4b code default: a G-rated boiler on code 102 (84%) is modelled at 66%, not
+    # the condensing 84% — the fix at the heart of #1704 / ADR-0068. "Override
+    # wins": the band-derived efficiency replaces the code's, symmetric up or down.
+    epc = _epc_with_override_boiler(None, (66.0, 56.0))  # G band (winter, summer)
+
+    inputs = cert_to_inputs(epc)
+
+    assert inputs.main_heating_efficiency == 0.66
+
+
+def test_no_override_keeps_the_table_4b_code_efficiency() -> None:
+    # Absent the slot, the code default stands (today's behaviour) — the override
+    # is the only thing that moves it.
+    epc = _epc_with_override_boiler(None, None)
+
+    inputs = cert_to_inputs(epc)
+
+    assert inputs.main_heating_efficiency == 0.84
+
+
+def test_seasonal_efficiency_override_feeds_the_eq_d1_water_summer_efficiency() -> None:
+    # The slot is a `(winter, summer)` pair; the summer leg drives SAP Appendix D
+    # §D2.1 Eq D1 water-heating efficiency (WHC 901, from main). A lower-band
+    # boiler burns less efficiently in summer, so it needs MORE hot-water fuel —
+    # a direction check that the override reaches the Eq D1 branch, not just §206.
+    default_code = _epc_with_override_boiler(None, None)  # code 102 → summer 74%
+    g_band = _epc_with_override_boiler(None, (66.0, 56.0))  # summer 56%
+
+    hw_default = cert_to_inputs(default_code).hot_water_kwh_per_yr
+    hw_g_band = cert_to_inputs(g_band).hot_water_kwh_per_yr
+
+    assert hw_g_band > hw_default
+
+
 def test_main_heating_efficiency_reads_sap_main_heating_code() -> None:
     # Arrange — Direction check: a gas combi (Table 4b code 102, 84% eff)
     # vs a non-condensing gas boiler (code 105, 70% eff) must show through