Map PasHub boiler PCDB product_id to main_heating_index_number

`_map_sap_heating` left `main_heating_index_number` None, so the
calculator fell to a generic Table-4b seasonal efficiency instead of the
actual appliance. The site note lodges the boiler by its PCDB `product_id`
(and `summer_efficiency` is always 0), and those ids resolve directly in
the calculator's PCDB (`gas_oil_boiler_record`) — so carrying `product_id`
onto `main_heating_index_number` gives the real SEDBUK efficiency.

Gated on `product_id > 0` (0 = no product lodged → stays None; the
calculator keeps its default). `main_heating_category`/`sap_main_heating_code`
were evaluated and add nothing measurable — the PCDB index alone is the
efficiency source.

Guinness GMCA cohort: within-0.5 12.9% -> 13.4%, MAE 2.657 -> 2.621, no
regression (harness still 206-xfail, no new hard failures).

Closes #1563

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

datatypes/epc/domain/mapper.py

@@ -6098,6 +6098,11 @@ def _map_sap_heating(
                 condensing=main.condensing,
                 weather_compensator=main.weather_compensator,
                 central_heating_pump_age_str=main.central_heating_pump_age,
+                # PCDB product id → the appliance's SEDBUK efficiency source
+                # (`gas_oil_boiler_record`); 0 means no product lodged (#1563).
+                main_heating_index_number=(
+                    main.product_id if main.product_id > 0 else None
+                ),
             )
         ],
         has_fixed_air_conditioning=ventilation.has_fixed_air_conditioning,

datatypes/epc/domain/tests/test_from_site_notes.py

@@ -245,6 +245,35 @@ class TestWaterHeatingCoding:
             )
 
 
+class TestMainHeatingEfficiencySource:
+    """`from_site_notes` must carry the surveyed PCDB `product_id` onto
+    `main_heating_index_number`, so the calculator reads the actual appliance's
+    SEDBUK efficiency (`gas_oil_boiler_record`) instead of a generic Table-4b
+    default. The site note lodges the boiler by PCDB id (`summer_efficiency` is
+    always 0), so the index IS the efficiency source (issue #1563).
+    """
+
+    def _survey(self, **main: object) -> PasHubRdSapSiteNotes:
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["heating_and_hot_water"]["main_heating"].update(main)
+        return from_dict(PasHubRdSapSiteNotes, data)
+
+    def test_product_id_maps_to_index_number(self) -> None:
+        result = EpcPropertyDataMapper.from_site_notes(self._survey(product_id=18400))
+        assert (
+            result.sap_heating.main_heating_details[0].main_heating_index_number
+            == 18400
+        )
+
+    def test_zero_product_id_leaves_index_none(self) -> None:
+        # No PCDB product lodged (e.g. selected by efficiency) → no index.
+        result = EpcPropertyDataMapper.from_site_notes(self._survey(product_id=0))
+        assert (
+            result.sap_heating.main_heating_details[0].main_heating_index_number
+            is None
+        )
+
+
 class TestFromSiteNotesExample1:
     """
     Fixture: pashub_rdsap_site_notes_example1.json
@@ -597,6 +626,7 @@ class TestFromSiteNotesExample1:
                         condensing=True,
                         weather_compensator=False,
                         central_heating_pump_age_str="Unknown",
+                        main_heating_index_number=18400,  # PCDB product id → efficiency source
                     )
                 ],
                 has_fixed_air_conditioning=False,