Include the C6 half heat-network standing charge on top of a water-only dwelling's fuel standing 🟩

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

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -2029,7 +2029,39 @@ def _standing_charges_total_gbp(
     water_heating_fuel_code: Optional[int],
     tariff: Tariff,
 ) -> float:
-    raise NotImplementedError
+    """Total annual standing charge (£/yr) for worksheet (251) — the single
+    source of truth for how the heat-network and fuel-based standing charges
+    compose.
+
+    SAP 10.2 §C6 (PDF p.54-55) settles the composition, and phrases its two
+    cases deliberately differently:
+
+    - Space heating on a heat network → "the TOTAL standing charge IS the
+      normal heat network standing charge" — a REPLACEMENT. The dwelling's own
+      fuel standing does not apply on top (see
+      `_heat_network_standing_charge_gbp`).
+    - DHW-only heat network → "INCLUDE one-half of the normal heat network
+      standing charge IN the calculation of fuel costs unless the space heating
+      is also a heat network" — ADDITIVE. Only the full case is worded as a
+      total, so the half stacks on whatever Table 12 note (a) already charges
+      the dwelling for its own space-heating fuel (e.g. £24 off-peak electric
+      for a storage-heater main → £84).
+
+    The DHW-only half keys off the WATER heating code (950 / 951 / 952), not
+    the space main: the space main is by definition NOT a network on this
+    branch, so gating on it (as the £120 branch does) can never fire here.
+    """
+    network_standing = _heat_network_standing_charge_gbp(epc, main)
+    if network_standing is not None:
+        return network_standing
+    fuel_standing = additional_standing_charges_gbp(
+        main_fuel_code=main_fuel_code,
+        water_heating_fuel_code=water_heating_fuel_code,
+        tariff=tariff,
+    )
+    if epc.sap_heating.water_heating_code in _WATER_HEAT_NETWORK_ONLY_CODES:
+        return fuel_standing + _HEAT_NETWORK_STANDING_CHARGE_GBP / 2.0
+    return fuel_standing
 
 
 def _main_heating_efficiency(epc: EpcPropertyData) -> float: