fix(mapper): read dropped detailed room-in-roof common-wall surfaces

Follow-on to the slope/stud slice. A Detailed RR (RdSAP 10 §3.9.2) can also
lodge `common_wall_*` — the wall separating the room-in-roof from the rest of
the cold roof void. Those fields were undeclared → `from_dict` dropped them →
`_api_rir_detailed_surfaces` omitted the common walls → the RR undercounted
wall heat loss → over-rate.

Fix: declare `common_wall_length/height_1/2` on `RoomInRoofDetails`
(21_0_0 + 21_0_1) and build `kind="common_wall"` surfaces (raw L × H area to
2 d.p.). The cascade's Detailed-RR branch already bills common walls at the
storey-below main-wall U (Table 4 p.22 "Common wall") and deducts their area
from the §3.10.1 residual roof — no calculator change. No insulation thickness
is read: common walls take the main-wall U, not a Table 17 RR-element U.

6 /tmp certs carry detailed `common_wall_length_1`: cohort mean|err| 2.43 ->
1.25 (all were over-rating; e.g. 2877-3059 +4.55 -> +2.79). Gauges: corpus
within-0.5 67.5% -> 67.6% (MAE 0.987 -> 0.979); /tmp 71.6% -> 71.7%
(MAE 0.846 -> 0.838). Harness 47/47 0 raised; regression = the 3 pre-existing
fails; pyright net-zero (65=65).

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

datatypes/epc/domain/mapper.py

@@ -3993,6 +3993,26 @@ def _api_rir_detailed_surfaces(
                         ),
                     )
                 )
+    # Common walls — billed as external wall at the storey-below main-wall U
+    # (cascade `kind="common_wall"`), so no insulation thickness is read.
+    # Detailed BPs use the raw L × H area (RdSAP 10 §3.9.2; the cascade's
+    # common_wall branch applies the L × (0.25 + H) form only to Simplified
+    # BPs). The cascade deducts this area from the §3.10.1 residual roof.
+    common_wall_specs = (
+        (details.common_wall_length_1, details.common_wall_height_1),
+        (details.common_wall_length_2, details.common_wall_height_2),
+    )
+    for length, height in common_wall_specs:
+        if (
+            length is not None and height is not None
+            and length > 0 and height > 0
+        ):
+            surfaces.append(
+                SapRoomInRoofSurface(
+                    kind="common_wall",
+                    area_m2=_round_half_up_2dp(float(length), float(height)),
+                )
+            )
     if (
         details.flat_ceiling_length_1 is not None
         and details.flat_ceiling_height_1 is not None

datatypes/epc/domain/tests/test_from_rdsap_schema.py

@@ -2202,22 +2202,29 @@ class TestRoomInRoofDetailedSlopeAndStudWall:
             "stud_wall_length_2": 7.0, "stud_wall_height_2": 1.03,
             "stud_wall_insulation_thickness_1": "75mm",
             "stud_wall_insulation_thickness_2": "75mm",
+            "common_wall_length_1": 8.6, "common_wall_height_1": 1.2,
+            "common_wall_length_2": 8.6, "common_wall_height_2": 1.2,
         }
 
         # Act
         result = EpcPropertyDataMapper.from_api_response(cert)
 
         # Assert — both slopes + both stud walls reach the cascade, with the
-        # lodged thickness parsed and the L × H area to 2 d.p.
+        # lodged thickness parsed and the L × H area to 2 d.p. Common walls
+        # route to the `common_wall` kind (raw L × H, billed at main-wall U).
         rir_part = result.sap_building_parts[0].sap_room_in_roof
         assert rir_part is not None
         surfaces = rir_part.detailed_surfaces
         assert surfaces is not None
         slopes = [s for s in surfaces if s.kind == "slope"]
         studs = [s for s in surfaces if s.kind == "stud_wall"]
+        commons = [s for s in surfaces if s.kind == "common_wall"]
         assert len(slopes) == 2
         assert len(studs) == 2
+        assert len(commons) == 2
         assert abs(slopes[0].area_m2 - 9.8) <= 1e-9
         assert slopes[0].insulation_thickness_mm == 100
         assert abs(studs[0].area_m2 - 7.21) <= 1e-9
         assert studs[0].insulation_thickness_mm == 75
+        assert abs(commons[0].area_m2 - 10.32) <= 1e-9
+        assert commons[0].insulation_thickness_mm is None

datatypes/epc/schema/rdsap_schema_21_0_0.py

@@ -228,6 +228,12 @@ class RoomInRoofDetails:
     stud_wall_insulation_type_2: Optional[int] = None
     stud_wall_insulation_thickness_1: Optional[str] = None
     stud_wall_insulation_thickness_2: Optional[str] = None
+    # §3.9.2 common walls of a Detailed RR — see
+    # `rdsap_schema_21_0_1.RoomInRoofDetails`. Previously dropped.
+    common_wall_length_1: Optional[float] = None
+    common_wall_length_2: Optional[float] = None
+    common_wall_height_1: Optional[float] = None
+    common_wall_height_2: Optional[float] = None
 
 
 @dataclass

datatypes/epc/schema/rdsap_schema_21_0_1.py

@@ -267,6 +267,15 @@ class RoomInRoofDetails:
     stud_wall_insulation_type_2: Optional[int] = None
     stud_wall_insulation_thickness_1: Optional[str] = None
     stud_wall_insulation_thickness_2: Optional[str] = None
+    # The §3.9.2 common walls of a Detailed RR (the wall separating the RR
+    # from the rest of the cold roof void). Billed as external wall at the
+    # storey-below main-wall U (cascade `kind="common_wall"`). Detailed BPs
+    # use the raw L × H area (Simplified Type-2 BPs use L × (0.25 + H)).
+    # Previously undeclared → dropped → the RR undercounted wall loss.
+    common_wall_length_1: Optional[float] = None
+    common_wall_length_2: Optional[float] = None
+    common_wall_height_1: Optional[float] = None
+    common_wall_height_2: Optional[float] = None
 
 
 @dataclass