feat(modelling): ASHP overlay resets water_heating_code to the HP end-state

The ASHP bundle is a fixed whole-system end-state (confirmed: always the same
contractor cylinder), so the hot-water arrangement is fixed too. The overlay now
sets water_heating_code=901 ("from main system") absolutely, so a combi (909/611)
or electric (903/908) before is reset to HW-from-the-heat-pump — previously the
overlay relied on the before already lodging 901 (true for boiler-1, not in
general). No-op for the boiler-1 pin (stays 1e-4). Cascade pins for combi /
electric-with-cylinder befores await example certs. ADR-0024.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

domain/modelling/generators/heating_recommendation.py

@@ -66,6 +66,11 @@ _ASHP_OVERLAY = HeatingOverlay(
     main_heating_control=2210,
     main_heating_index_number=101413,
     main_heating_category=_HEAT_PUMP_CATEGORY,
+    # Hot water from the main heat-pump system via the new cylinder (code 901,
+    # "from main system"). Set absolutely so a combi (909/611) or electric
+    # (903/908) before is reset to the fixed HP end-state, not just the case
+    # where the before already lodged 901.
+    water_heating_code=901,
     water_heating_fuel=_ELECTRICITY_FUEL,
     cylinder_size=4,
     cylinder_insulation_type=1,

tests/domain/modelling/test_heating_recommendation.py

@@ -154,6 +154,7 @@ def test_gas_boiler_house_yields_an_ashp_bundle() -> None:
         main_heating_control=2210,
         main_heating_index_number=101413,
         main_heating_category=4,
+        water_heating_code=901,
         water_heating_fuel=30,
         cylinder_size=4,
         cylinder_insulation_type=1,