Slice S0380.62: wire Table 32 standing charges into the off-peak cost fallback

The cascade's `additional_standing_charges_gbp(main_fuel_code,
water_heating_fuel_code, tariff)` function (table_32.py:178) was
already producing the right values — for cert 000565 it returns
£143 (£120 mains gas standing + £23 10-hour high-rate electricity
standing per Table 32 page 95). But the value only landed in
`FuelCostResult.additional_standing_charges_gbp` inside `_fuel_cost`,
which returns `_ZERO_FUEL_COST_FOR_OFF_PEAK` for non-STANDARD
tariff. The calculator then falls back to the inline cost math
(scalar fuel-cost × kWh) which had no standing-charge component →
£143 was silently dropped from the off-peak cost cascade.

New `CalculatorInputs.standing_charges_gbp: float = 0.0` field
carries the standing-charge total into the fallback path. The
inline cost summation adds it before max-clamp + PV credit.
STANDARD-tariff certs route via `fuel_cost.additional_standing_
charges_gbp` (set inside `_fuel_cost`) and the calculator ignores
this scalar on that path — no double-count.

`cert_to_inputs` populates the new field unconditionally; the value
is just zero on standard-tariff certs (Table 12 note (a) gates
standing-charge inclusion regardless).

Cert 000565 cascade impact:
- standing_charges_gbp = £143.00 ✓ (exact match to worksheet line 251)
- total_fuel_cost_gbp:  Δ −310 → −167  (46% reduction)
- sap_score_continuous: Δ +3.61 → +1.91 (47% reduction)
- co2_kg_per_yr:        Δ unchanged (standing charges don't bill CO2)

Cohort regression check: 427 pass + 10 expected 000565 fails. The
14 existing Elmhurst fixtures + JSON fixtures all have meter_type=
None → STANDARD → standing routes via FuelCostResult unchanged.

Spec source: RdSAP 10 Table 32 page 95 standing-charge column;
SAP 10.2 Table 12 note (a) inclusion gating.

Pyright net-zero on both files (0 / 34).

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

domain/sap10_calculator/calculator.py

@@ -300,6 +300,13 @@ class CalculatorInputs:
     fuel_cost: FuelCostResult = field(
         default_factory=lambda: _ZERO_FUEL_COST_RESULT
     )
+    # Table 32 standing charges (electric off-peak high-rate code +
+    # mains gas) — added to `total_cost` when the calculator's off-
+    # peak fallback path fires. STANDARD-tariff certs route through
+    # `fuel_cost.additional_standing_charges_gbp` instead and ignore
+    # this field. cert_to_inputs sets this via `additional_standing_
+    # charges_gbp(main_fuel_code, water_heating_fuel_code, tariff)`.
+    standing_charges_gbp: float = 0.0
 
 
 @dataclass(frozen=True)
@@ -518,6 +525,7 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
             + hot_water_cost
             + pumps_fans_cost
             + lighting_cost
+            + inputs.standing_charges_gbp
             - pv_credit,
         )
     ecf = energy_cost_factor(total_cost_gbp=total_cost, total_floor_area_m2=tfa)

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -3405,6 +3405,15 @@ def cert_to_inputs(
         other_fuel_cost_gbp_per_kwh=_other_fuel_cost_gbp_per_kwh(
             _rdsap_tariff(epc), prices
         ),
+        # Table 32 standing charges for the off-peak fallback path.
+        # STANDARD-tariff certs route via `fuel_cost.additional_
+        # standing_charges_gbp` (set inside `_fuel_cost`) and the
+        # calculator ignores this scalar on that path.
+        standing_charges_gbp=additional_standing_charges_gbp(
+            main_fuel_code=_main_fuel_code(main),
+            water_heating_fuel_code=_water_heating_fuel_code(epc),
+            tariff=_rdsap_tariff(epc),
+        ),
         co2_factor_kg_per_kwh=_co2_factor_kg_per_kwh(main),
         # SAP10.2 Table 12d (p.194) per-end-use effective CO2 factors. For
         # electricity end-uses Σ(kWh_m × CO2_m) / Σ(kWh_m) replaces the