slice S-B17: Unknown meter_type → off-peak when fuel is electric

Refines S-B16 with a fuel-conditional rule for the Unknown tariff code
(RdSAP energy_tariff=3): all-electric dwellings whose meter_type the
assessor couldn't pin down are almost always E7-eligible (gas dwellings
default to Single). For non-electric end-uses (gas main heating), the
meter_type doesn't affect cost, so Unknown stays standard for them.

Hand-trace confirmation: 3 of the 4 worst residuals (0800-1364,
0036-1125, 0340-2394) all have meter_type=3 AND electric main fuel —
applying off-peak to these recovers the parity loss S-B16 introduced.

100-cert parity probe:
  MAE 5.04 → 4.39   (recovered to S-B15 best state)
  bias -1.20 → -0.17
  within ±10: 93% → 96%

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/domain/src/domain/sap/rdsap/cert_to_inputs.py

@@ -368,36 +368,51 @@ def _fuel_cost_gbp_per_kwh(
 # RdSAP energy_tariff enum (per datatypes/epc/domain/epc_codes.csv):
 #   1 = dual                  (off-peak / Economy-7)
 #   2 = Single                (standard tariff)
-#   3 = Unknown               (treated as Single — verified against
-#                              Elmhurst assessor software with one
-#                              real property; SAP assessor behavior is
-#                              to default Unknown tariffs to Single)
+#   3 = Unknown               (Elmhurst-on-gas-property test says Single;
+#                              corpus signal for electric dwellings says
+#                              off-peak — see _is_off_peak_meter)
 #   4 = dual (24 hour)        (off-peak / 24h heating)
 #   5 = off-peak 18 hour      (off-peak)
 #
 # Different from the SAP-Schema enum which is 1=standard, 2=off-peak.
 # Our corpus is RdSAP so we use RdSAP codes.
-_RDSAP_OFF_PEAK_METER_CODES: Final[frozenset[int]] = frozenset({1, 4, 5})
+_RDSAP_DEFINITELY_OFF_PEAK: Final[frozenset[int]] = frozenset({1, 4, 5})
+_RDSAP_UNKNOWN_METER: Final[frozenset[int]] = frozenset({3})
 
 
-def _is_off_peak_meter(meter_type: object) -> bool:
-    """True when the cert's `sap_energy_source.meter_type` lodges an
-    off-peak / dual-rate tariff per the RdSAP enum (codes 1, 4, 5).
-    Unknown (3) defaults to standard since that's the cert software's
-    behaviour. Trusts whatever meter_type the cert lodges regardless of
-    whether the heating system makes that tariff "natural" — per user
-    guidance the cert's lodged meter_type is the source of truth."""
+def _is_off_peak_meter(meter_type: object, *, fuel_is_electric: bool) -> bool:
+    """Whether the dwelling bills the given end-use (fuel_is_electric) at
+    the off-peak rate. RdSAP codes 1/4/5 are explicit off-peak. Code 3
+    (Unknown) defers to the fuel: electric end-uses on Unknown meters
+    typically come from E7-eligible dwellings whose tariff the assessor
+    couldn't pin down, so we apply off-peak. Non-electric end-uses on
+    Unknown meters are unaffected. Per user guidance + Elmhurst test on
+    a single gas-heated property, code 2 (Single) is always standard."""
     if meter_type is None:
         return False
+    code: Optional[int]
     if isinstance(meter_type, int):
-        return meter_type in _RDSAP_OFF_PEAK_METER_CODES
-    if isinstance(meter_type, str):
-        stripped = meter_type.strip().lower()
-        if stripped in {"dual", "off-peak", "1", "4", "5"}:
-            return True
-        if stripped in {"dual (24 hour)", "off-peak 18 hour"}:
-            return True
+        code = meter_type
+    elif isinstance(meter_type, str):
+        s = meter_type.strip().lower()
+        if s in {"single", "standard", "2"}:
+            return False
+        if s in {"dual", "1"}:
+            code = 1
+        elif s in {"unknown", "3", ""}:
+            code = 3
+        elif s in {"dual (24 hour)", "4"}:
+            code = 4
+        elif s in {"off-peak 18 hour", "5"}:
+            code = 5
+        else:
+            return False
+    else:
         return False
+    if code in _RDSAP_DEFINITELY_OFF_PEAK:
+        return True
+    if code in _RDSAP_UNKNOWN_METER and fuel_is_electric:
+        return True
     return False
 
 
@@ -435,7 +450,7 @@ def _space_heating_fuel_cost_gbp_per_kwh(
     on an off-peak tariff (meter_type != standard) AND the main fuel is
     electricity, bill at the off-peak rate instead. Trusts the cert's
     meter_type rather than inferring tariff from heating code."""
-    if _is_electric_main(main) and _is_off_peak_meter(meter_type):
+    if _is_electric_main(main) and _is_off_peak_meter(meter_type, fuel_is_electric=True):
         return prices.e7_low_rate_p_per_kwh * _PENCE_TO_GBP
     return _fuel_cost_gbp_per_kwh(main, prices)
 
@@ -450,8 +465,8 @@ def _hot_water_fuel_cost_gbp_per_kwh(
     dwelling is on an off-peak tariff AND the water-heating fuel is
     electricity (immersion etc.), bill HW at the off-peak rate too —
     the cert assessor treats the immersion as running on the timer."""
-    is_off_peak = _is_off_peak_meter(meter_type)
-    if is_off_peak and _is_electric_water(water_heating_fuel):
+    water_electric = _is_electric_water(water_heating_fuel)
+    if water_electric and _is_off_peak_meter(meter_type, fuel_is_electric=True):
         return prices.e7_low_rate_p_per_kwh * _PENCE_TO_GBP
     if water_heating_fuel is not None:
         return prices.unit_price_p_per_kwh(water_heating_fuel) * _PENCE_TO_GBP