P6.1 follow-on: use BuildingPartIdentifier enum in ml/transform + tests

Replace the string literal "Main Dwelling" / "Extension 1" comparisons
in `_building_part_aggregates` and the four affected tests with the
typed `BuildingPartIdentifier.MAIN` / `.EXTENSION_1` enum values, so
the transform is consistent with the typed domain introduced in the P6.1
cert→inputs adapter. Fixes a latent mismatch that would silently return
`main=None` if the string ever drifted.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/domain/src/domain/ml/tests/test_transform.py

@@ -3,7 +3,11 @@
 import pandas as pd
 import pytest
 
-from datatypes.epc.domain.epc_property_data import SapRoomInRoof, WindowTransmissionDetails
+from datatypes.epc.domain.epc_property_data import (
+    BuildingPartIdentifier,
+    SapRoomInRoof,
+    WindowTransmissionDetails,
+)
 from domain.ml.schema import ColumnSpec, TransformSchema
 from domain.ml.tests._fixtures import (
     make_building_part,
@@ -540,7 +544,7 @@ def test_schema_advertises_building_part_features() -> None:
 def test_to_row_aggregates_building_parts_with_main_dwelling_carveout() -> None:
     # Arrange — Main Dwelling (two floors, age band B, wall 3, roof 4) plus one extension.
     main = make_building_part(
-        identifier="Main Dwelling",
+        identifier=BuildingPartIdentifier.MAIN,
         construction_age_band="B",
         wall_construction=3,
         roof_construction=4,
@@ -556,7 +560,7 @@ def test_to_row_aggregates_building_parts_with_main_dwelling_carveout() -> None:
         ],
     )
     extension = make_building_part(
-        identifier="Extension 1",
+        identifier=BuildingPartIdentifier.EXTENSION_1,
         construction_age_band="L",
         wall_construction=4,
         roof_construction=5,
@@ -600,7 +604,7 @@ def test_to_row_aggregates_building_parts_with_main_dwelling_carveout() -> None:
 def test_to_row_flags_room_in_roof_when_main_dwelling_has_it() -> None:
     # Arrange
     main = make_building_part(
-        identifier="Main Dwelling",
+        identifier=BuildingPartIdentifier.MAIN,
         sap_room_in_roof=SapRoomInRoof(floor_area=15.0, construction_age_band="B"),
     )
     epc = make_minimal_sap10_epc(energy_rating_current=82, sap_building_parts=[main])
@@ -615,7 +619,7 @@ def test_to_row_flags_room_in_roof_when_main_dwelling_has_it() -> None:
 
 def test_to_row_returns_building_part_nones_when_no_main_dwelling_identified() -> None:
     # Arrange — single part with identifier that doesn't match "Main Dwelling"
-    sole_part = make_building_part(identifier="Extension 1")
+    sole_part = make_building_part(identifier=BuildingPartIdentifier.EXTENSION_1)
     epc = make_minimal_sap10_epc(
         energy_rating_current=82, sap_building_parts=[sole_part]
     )
@@ -1188,7 +1192,7 @@ def test_to_row_extracts_main_dwelling_wall_roof_floor_fabric_inputs() -> None:
         floor=1, floor_insulation=0, floor_construction=0,
     )
     main = SapBuildingPart(
-        identifier="Main Dwelling",
+        identifier=BuildingPartIdentifier.MAIN,
         construction_age_band="C",
         wall_construction=3,
         wall_insulation_type=4,
@@ -1229,7 +1233,7 @@ def test_to_row_parses_no_insulation_sentinel_as_zero_mm() -> None:
     # Arrange
     from datatypes.epc.domain.epc_property_data import SapBuildingPart
     main = SapBuildingPart(
-        identifier="Main Dwelling",
+        identifier=BuildingPartIdentifier.MAIN,
         construction_age_band="C",
         wall_construction=3,
         wall_insulation_type=4,
@@ -1268,7 +1272,7 @@ def test_to_row_emits_positive_envelope_heat_loss_for_sap10_epc() -> None:
     from domain.ml.tests._fixtures import make_building_part, make_floor_dimension
 
     main = make_building_part(
-        identifier="Main Dwelling",
+        identifier=BuildingPartIdentifier.MAIN,
         construction_age_band="G",
         wall_construction=4,
         wall_insulation_type=4,

packages/domain/src/domain/ml/transform.py

@@ -16,6 +16,7 @@ import pandas as pd
 
 from datatypes.epc.domain.epc import Epc
 from datatypes.epc.domain.epc_property_data import (
+    BuildingPartIdentifier,
     EnergyElement,
     EpcPropertyData,
     SapBuildingPart,
@@ -41,9 +42,6 @@ from domain.ml.schema import ColumnSpec, TransformSchema
 from domain.ml.ucl import apply_ucl_correction
 
 
-_MAIN_DWELLING_IDENTIFIER = "Main Dwelling"
-
-
 # SAP10 orientation codes: 1=N, 2=NE, 3=E, 4=SE, 5=S, 6=SW, 7=W, 8=NW.
 # Anything else (0, "NR", etc.) is treated as unrecorded — it contributes to
 # `window_count` and `window_total_area_m2` but to no octant.
@@ -1477,7 +1475,7 @@ def _building_part_aggregates(parts: list[SapBuildingPart]) -> dict[str, Any]:
     present — otherwise None (we don't silently fall back to the first part).
     """
     main = next(
-        (p for p in parts if p.identifier == _MAIN_DWELLING_IDENTIFIER), None
+        (p for p in parts if p.identifier is BuildingPartIdentifier.MAIN), None
     )
     aggregates: dict[str, Any] = {
         "building_parts_count": len(parts),
@@ -1588,7 +1586,7 @@ def _building_part_aggregates(parts: list[SapBuildingPart]) -> dict[str, Any]:
 
     # Extension 1 — first non-main entry in the list.
     secondary = next(
-        (p for p in parts if p.identifier != _MAIN_DWELLING_IDENTIFIER), None
+        (p for p in parts if p.identifier is not BuildingPartIdentifier.MAIN), None
     )
     if secondary is not None:
         aggregates["extension_1_present"] = True