Synthesise glazing from the glazed_area band for windowless 18.0 certs 🟩

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

datatypes/epc/domain/mapper.py

@@ -852,7 +852,9 @@ class EpcPropertyDataMapper:
                     permanent_shutters_present=False,
                 )
                 for w in schema.sap_windows
-            ],
+            ]
+            if schema.sap_windows
+            else _synthesise_18_0_sap_windows(schema),
             sap_energy_source=SapEnergySource(
                 mains_gas=es.mains_gas == "Y",
                 meter_type=str(es.meter_type),
@@ -3007,6 +3009,42 @@ def _synthesise_20_0_0_sap_windows(schema: RdSapSchema20_0_0) -> List[SapWindow]
     ]
 
 
+def _synthesise_18_0_sap_windows(schema: RdSapSchema18_0) -> List[SapWindow]:
+    """ADR-0028 Reduced-Field Synthesis of `sap_windows` for an 18.0 cert.
+
+    A separate seam from the 20.0.0 helper so 18.0 can diverge as we learn more,
+    but it reuses the inherited 20.0.0 coefficients unchanged (ADR-0028: the
+    18.0 corpus can't self-fit — 958/1000 band-1 with no measured band-1 windows
+    — and its band-4 rich certs reproduce `0.148 x 1.51 = 0.223`). 990/1000 certs
+    carry no per-window array, only a glazed_area band + floor area; synthesise
+    total glazing as `ratio x TFA`, split 4-way across N/E/S/W with
+    `window_width = area/4, window_height = 1.0`.
+    """
+    band_multiplier = _RDSAP20_GLAZED_AREA_BAND_MULTIPLIER.get(schema.glazed_area, 1.0)
+    total_area = (
+        _RDSAP20_GLAZING_RATIO * float(schema.total_floor_area) * band_multiplier
+    )
+    per_window_area = total_area / len(_RDSAP20_SYNTH_ORIENTATIONS)
+    return [
+        SapWindow(
+            frame_material=None,
+            glazing_gap=0,
+            orientation=orientation,
+            window_type=0,
+            # ADR-0028: 18.0 glazed_type codes 1-8+ND are identical to 20.0.0's
+            # (verified against epc_codes.csv), so reuse the verified cascade.
+            glazing_type=_api_cascade_glazing_type(schema.multiple_glazing_type),
+            window_width=per_window_area,
+            window_height=1.0,
+            draught_proofed=False,
+            window_location=0,
+            window_wall_type=0,
+            permanent_shutters_present=False,
+        )
+        for orientation in _RDSAP20_SYNTH_ORIENTATIONS
+    ]
+
+
 # GOV.UK API `glazing_type` integer → (u_value W/m²K, g_perpendicular,
 # frame_factor) lookup the cascade reads via `window_transmission_
 # details` for per-window cascade fidelity. The cascade defaults to a