Derive hot-water bath and mixer counts from room counts for 18.0 certs 🟩

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

datatypes/epc/domain/mapper.py

@@ -743,6 +743,8 @@ class EpcPropertyDataMapper:
     @staticmethod
     def from_rdsap_schema_18_0(schema: RdSapSchema18_0) -> EpcPropertyData:
         es = schema.sap_energy_source
+        # ADR-0028: instantaneous_wwhrs holds bath/shower ROOM counts.
+        iw = schema.sap_heating.instantaneous_wwhrs
         return EpcPropertyData(
             uprn=schema.uprn,
             assessment_type=schema.assessment_type,
@@ -809,6 +811,20 @@ class EpcPropertyDataMapper:
             ),
             sap_heating=SapHeating(
                 instantaneous_wwhrs=InstantaneousWwhrs(),
+                # ADR-0028: derive HW demand counts from the bath/shower
+                # ROOM counts (instantaneous_wwhrs is the false-friend container).
+                number_baths=(
+                    iw.rooms_with_bath_and_or_shower
+                    + iw.rooms_with_bath_and_mixer_shower
+                    if iw is not None
+                    else None
+                ),
+                mixer_shower_count=(
+                    iw.rooms_with_mixer_shower_no_bath
+                    + iw.rooms_with_bath_and_mixer_shower
+                    if iw is not None
+                    else None
+                ),
                 main_heating_details=[
                     MainHeatingDetail(
                         has_fghrs=d.has_fghrs == "Y",