Slice 60: thermal bridging y is dwelling-wide, not per-bp

`heat_transmission_from_cert` computed `y = thermal_bridging_y(age_
band=part.construction_age_band)` per bp, then applied each bp's y
to its own external area. That mis-models multi-age dwellings:
RdSAP10 Table 21 indexes y by the *dwelling's* age band, and Elmhurst's
worksheet reports y as a single user-defined value applied to total
exposed area (cert 001479 worksheet: "Thermal Bridges Bridging User
Input Y 0.15").

For cohort certs with uniform age-band bps the change is heat-loss-
invariant. For cert 001479 (Main=C → 0.15, Ext1=M → 0.08, Ext2=C →
0.15) the cascade was under-counting Ext1's bridging by 0.07 × 27.28
m² ≈ 1.9 W/K. For golden cert 7536-3827 (Main=D, Ext1=L, Ext2=F) the
same per-bp split was costing ~2 W/K of bridging.

Use the primary part's (parts[0]) age band for a single dwelling-wide
`dwelling_y`, applied across all parts in the heat-loss loop.

Cert 001479 chain pin closes another step: cascade SAP 70.38 → 70.20
(target 69.0094, delta 1.37 → 1.19). Golden 7536-3827 residuals
tighten in lockstep: SAP +4 → +3, PE -24.73 → -22.53, CO2 -0.66 → -0.60.
Other 7 golden certs unchanged (single-bp or uniform-age multi-bp).

70 of 71 chain+golden+heat-transmission tests green; chain pin still
RED (load-bearing). Pyright net-zero (13-error baseline on
heat_transmission.py preserved).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/domain/src/domain/sap/rdsap/tests/test_golden_fixtures.py

@@ -132,13 +132,15 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
     _GoldenExpectation(
         cert_number="7536-3827-0600-0600-0276",
         actual_sap=68,
-        expected_sap_resid=+4,
-        expected_pe_resid_kwh_per_m2=-24.7328,
-        expected_co2_resid_tonnes_per_yr=-0.6580,
+        expected_sap_resid=+3,
+        expected_pe_resid_kwh_per_m2=-22.5292,
+        expected_co2_resid_tonnes_per_yr=-0.5993,
         notes=(
-            "Detached + 2 extensions, TFA 152, age D, gas PCDB. Slice 59 "
-            "per-bp window apportionment tightens PE -27.17 → -24.73 and "
-            "CO2 -0.72 → -0.66; SAP residual unchanged at +4."
+            "Detached + 2 extensions, TFA 152. Multi-age bps (Main=D, "
+            "Ext1=L, Ext2=F). Slice 59 (per-bp window apportionment) and "
+            "Slice 60 (dwelling-wide thermal bridging y from primary bp's "
+            "age band, not per-bp) jointly tightened: SAP +4 → +3, PE "
+            "-27.17 → -22.53, CO2 -0.72 → -0.60."
         ),
     ),
     _GoldenExpectation(

packages/domain/src/domain/sap/worksheet/heat_transmission.py

@@ -395,6 +395,17 @@ def heat_transmission_from_cert(
     bridging = 0.0
     total_external_area = 0.0
 
+    # RdSAP10 Table 21 — thermal-bridging factor `y` is keyed off the
+    # *dwelling's* age band (typically the Main part's), not per bp.
+    # Elmhurst's worksheet reports y as a single user-defined value
+    # applied to total exposed area (see worksheet row "Thermal Bridges
+    # Bridging User Input Y 0.15"). For multi-bp dwellings with mixed-age
+    # extensions (cert 001479: Main=C, Ext1=M, Ext2=C), applying per-bp
+    # y mis-models Ext1's bridging at 0.08 instead of 0.15 — a 0.07 ×
+    # 27 m² ≈ 1.9 W/K under-count on this cert.
+    primary_age_band = parts[0].construction_age_band
+    dwelling_y = thermal_bridging_y(age_band=primary_age_band)
+
     # Pre-compute per-bp window areas so each bp's gross wall is reduced by
     # only the openings physically cut into it. Previously every window
     # was apportioned to part i==0 (Main); that's heat-loss-invariant when
@@ -493,7 +504,10 @@ def heat_transmission_from_cert(
                 description=floor_description,
             )
         upw = u_party_wall(party_wall_construction=party_construction)
-        y = thermal_bridging_y(age_band=age_band)
+        # Per-bp `y` for backwards compat: when the bp's own age band
+        # differs from the dwelling's primary, the cascade applies the
+        # dwelling-wide value (RdSAP10 Table 21 convention).
+        y = dwelling_y
 
         # RdSAP10 §15 — element gross areas enter the SAP calculator at
         # 2 d.p. precision. `_part_geometry` rounds gross wall + party