fix(modelling): HHR overlay sets off-peak immersion type so HW Table 13 applies

The HHR-storage HeatingOverlay (ADR-0024) added an off-peak electric
immersion cylinder but never set `immersion_heating_type`, so the overlaid
cert left it None. The calculator then could not resolve `immersion_single`
for the SAP 10.2 Table 13 HW high-rate split and billed hot water 100% at
the off-peak low rate — £127.41 vs the relodged after-cert's £169.39,
overstating the overlay's SAP by +1.26 (CO2/PE matched, isolating it to the
HW cost path).

Add `immersion_heating_type` to HeatingOverlay, route it through
`_fold_heating` (it lives on `sap_heating`), and set it to 1 (single
off-peak immersion) on the HHR overlay to match the relodged reference.
Closes both `test_hhr_storage_overlay_reproduces_the_relodged_after_*`
cascade pins (electric-storage and no-system befores share the after).

Pre-existing failure (present before this branch's recent commits), outside
the handover regression gate. Full modelling suite 220 pass, pyright net-
zero.

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

domain/modelling/generators/heating_recommendation.py

@@ -64,6 +64,10 @@ _HHR_STORAGE_OVERLAY = HeatingOverlay(
     cylinder_insulation_type=1,
     cylinder_insulation_thickness_mm=120,
     cylinder_thermostat="Y",
+    # Single off-peak electric immersion — drives the SAP 10.2 Table 13 HW
+    # high-rate split (matches the relodged after-cert; without it the HW
+    # bills 100% at the low rate, +1.26 SAP over the reference).
+    immersion_heating_type=1,
     has_hot_water_cylinder=True,
     meter_type="Dual",
 )

domain/modelling/scoring/overlay_applicator.py

@@ -98,6 +98,7 @@ _SAP_HEATING_FIELDS: tuple[str, ...] = (
     "cylinder_insulation_type",
     "cylinder_insulation_thickness_mm",
     "cylinder_thermostat",
+    "immersion_heating_type",
 )
 _ENERGY_SOURCE_FIELDS: tuple[str, ...] = ("meter_type", "mains_gas")
 

domain/modelling/simulation.py

@@ -133,6 +133,11 @@ class HeatingOverlay:
     cylinder_insulation_type: Optional[Union[int, str]] = None
     cylinder_insulation_thickness_mm: Optional[int] = None
     cylinder_thermostat: Optional[str] = None
+    # The cylinder's immersion-heater arrangement (e.g. single off-peak
+    # immersion = 1). Drives the SAP 10.2 Table 13 HW high-rate fraction on
+    # off-peak tariffs — without it the calculator cannot resolve
+    # `immersion_single` and bills HW 100% at the low rate.
+    immersion_heating_type: Optional[Union[int, str]] = None
     # EpcPropertyData (top-level)
     has_hot_water_cylinder: Optional[bool] = None
     # sap_energy_source

tests/domain/modelling/test_heating_recommendation.py

@@ -67,6 +67,7 @@ def test_electric_storage_dwelling_yields_an_hhr_storage_bundle() -> None:
         cylinder_insulation_type=1,
         cylinder_insulation_thickness_mm=120,
         cylinder_thermostat="Y",
+        immersion_heating_type=1,
         has_hot_water_cylinder=True,
         meter_type="Dual",
     )