Slice 22: per-window curtain resistance — fixes mixed-glazing window U

SAP 10.2 §3.2 applies the 0.04 m²K/W curtain resistance per window;
the worksheet's (27) column shows it that way. Our calc had been
applying it ONCE to the area-weighted-avg raw U across all windows.
That's correct when all windows share a U but biased when a dwelling
has mixed glazing types (typical Elmhurst fixture lodges 2 types):

  U_eff(weighted_avg(U_i)) ≠ weighted_avg(U_eff(U_i))

because 1/(1/U + 0.04) is non-linear. The drift was ~0.05-0.10 W/K
on `windows_w_per_k` for 000474, 000477, 000487 (mixed-glazing
fixtures).

Fix: when sap_windows have per-window u_value lodged (the spec-
faithful path), iterate them computing per-window U_eff × area and
sum. Falls back to the legacy single-avg-U path when window U isn't
lodged (back-compat for synthetic tests that pass
`window_avg_u_value=...` directly).

Per-window LINE_27 numbers now match PDF exactly:

  fixture | windows W/K calc → PDF | LINE_33 Δ before → after
  --------|------------------------|---------------------------
  000474  | 25.4243 → 25.3674 ✓    |   +0.0864 → +0.0296  (-66%)
  000477  | 17.8550 → 17.8349 ✓    |   -0.1045 → -0.1246  (small
                                       widening — exposes
                                       upstream floor-U drift)
  000487  | (cascading)            |   +37.88 (RR defect, slice 23)
  000480  | unchanged              |   -0.0168 → -0.0168  (single U)
  000490  | unchanged              |   +0.0282 → +0.0282  (single U)
  000516  | (cascading)            |   -6.75 (RR defect, slice 23)

Total cascade pin failure count unchanged at 83 (pins still above
abs=1e-4 floor by 0.03-0.13 W/K — sub-display-precision drift left
in floor-U cascades + the two RR fixture defects).

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

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

@@ -263,17 +263,37 @@ def heat_transmission_from_cert(
     floor_description = _joined_descriptions(epc.floors)
 
     door_area = max(0, door_count) * _DEFAULT_DOOR_AREA_M2
-    window_u_raw = window_avg_u_value if (window_avg_u_value or 0) > 0 else u_window(
-        installed_year=None, glazing_type=None, frame_type=None
-    )
     # SAP10.2 §3.2: effective window U includes the 0.04 m²K/W curtain
-    # resistance — matches the (27) column in the worksheet (raw U=2.0
-    # → effective 1/(0.5+0.04)=1.852).
-    window_u = (
-        1.0 / (1.0 / window_u_raw + _WINDOW_CURTAIN_RESISTANCE_M2K_PER_W)
-        if window_u_raw > 0
-        else 0.0
+    # resistance — `(27)` worksheet column applies it per-window. When
+    # sap_windows have per-window U lodgements (mixed glazing types in
+    # the same dwelling), per-window curtain transform is the spec-
+    # faithful path: U_eff(weighted_avg) ≠ weighted_avg(U_eff(U_i)) due
+    # to the curtain resistance non-linearity.
+    windows_have_per_window_u = bool(epc.sap_windows) and all(
+        w.window_transmission_details is not None
+        and w.window_transmission_details.u_value is not None
+        for w in (epc.sap_windows or [])
     )
+    if windows_have_per_window_u:
+        windows_w_per_k_total = 0.0
+        for w in epc.sap_windows or []:
+            a_w = float(w.window_width) * float(w.window_height)
+            u_raw_w = float(w.window_transmission_details.u_value)  # type: ignore[union-attr]
+            u_eff_w = (
+                1.0 / (1.0 / u_raw_w + _WINDOW_CURTAIN_RESISTANCE_M2K_PER_W)
+                if u_raw_w > 0 else 0.0
+            )
+            windows_w_per_k_total += a_w * u_eff_w
+    else:
+        window_u_raw = window_avg_u_value if (window_avg_u_value or 0) > 0 else u_window(
+            installed_year=None, glazing_type=None, frame_type=None
+        )
+        window_u = (
+            1.0 / (1.0 / window_u_raw + _WINDOW_CURTAIN_RESISTANCE_M2K_PER_W)
+            if window_u_raw > 0
+            else 0.0
+        )
+        windows_w_per_k_total = window_u * window_total_area_m2
     primary_age = parts[0].construction_age_band
     door_uninsulated_u = u_door(country=country, age_band=primary_age, insulated=False, insulated_u_value=None)
     door_insulated_u = (
@@ -288,7 +308,9 @@ def heat_transmission_from_cert(
     roof = 0.0
     floor = 0.0
     party = 0.0
-    windows = 0.0
+    windows = windows_w_per_k_total  # SAP10.2 §3.2 — total computed
+                                     # pre-loop with correct per-window
+                                     # curtain transform.
     doors = 0.0
     bridging = 0.0
     total_external_area = 0.0
@@ -449,7 +471,8 @@ def heat_transmission_from_cert(
                     party += 0.25 * area
         floor += uf * floor_area_total
         party += upw * party_area
-        windows += window_u * w_area
+        # windows: total computed pre-loop (`windows_w_per_k_total`).
+        # w_area still drives the net-wall opening subtraction below.
         doors += door_u * d_area
         # (31) — total external element area used by both the worksheet
         # readout and the (36) thermal-bridging multiplier. Excludes the