Dispatch PasHub mechanical ventilation into the §2 ACH cascade 🟩

The surveyed "Ventilation type" label never reached
mechanical_ventilation_kind, so ventilation_from_cert silently defaulted every
mechanical system to NATURAL (24d), dropping its ACH heat loss — 76/205
cohort fixtures lodge dMEV/MEV/PIV (issue #1590 bug 4). Mapping mirrors the
gov-API _API_MECHANICAL_VENTILATION_TO_KIND table (PIV-from-loft is
"as natural"). Harness: within-0.5 12.2% -> 18.5%, MAE 2.538 -> 1.730 — the
campaign's largest single move; floors ratcheted to 0.18 / 1.75.

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

backend/documents_parser/tests/test_pashub_sap_accuracy.py

@@ -80,7 +80,11 @@ _MANIFEST_PATH = _FIXTURES_DIR / "manifest.json"
 # Both are tracked for verification (pre_sap correctness vs a deeper modelling
 # gap) — see docs/HANDOVER_PASHUB_838_PROBLEM_PROPERTIES.md. Re-baselined to the
 # full-coverage aggregate; ratchet back DOWN as those two are resolved.
-_MIN_WITHIN_HALF: float = 0.12
+# Ratcheted 0.12 -> 0.18 / 2.55 -> 1.75 by #1590 bug 4 (mechanical ventilation
+# dispatched into the §2 ACH cascade instead of defaulting to NATURAL): 76/205
+# cohort fixtures lodge a mechanical system; observed 18.5% within-0.5,
+# MAE 1.730 — the campaign's largest single move.
+_MIN_WITHIN_HALF: float = 0.18
 # Ratcheted 2.71 -> 2.64 by #1590 bug 2 (roof "Insulation At: None" -> explicit
 # zero thickness): observed MAE 2.632 — the two ~7-SAP roof over-raters
 # (507665533138 / 507670893756) correcting is exactly 14/205 ≈ 0.07 of MAE.
@@ -89,7 +93,7 @@ _MIN_WITHIN_HALF: float = 0.12
 # under-rate closing.
 # Ratcheted 2.58 -> 2.55 by #1590 bug 3 (PV Connection label -> gov-API int:
 # "Not connected" no longer silently credited): observed MAE 2.538.
-_MAX_SAP_MAE: float = 2.55
+_MAX_SAP_MAE: float = 1.75
 
 _KNOWN_GAP_REASON = (
     "pashub `from_site_notes` mapper does not int-code a main-heating "

datatypes/epc/domain/mapper.py

@@ -6142,9 +6142,42 @@ def _map_sap_heating(
     )
 
 
+# PasHub surveyed "Ventilation type" label → `MechanicalVentilationKind` enum
+# name for the §2 cascade dispatch, mirroring the gov-API table
+# `_API_MECHANICAL_VENTILATION_TO_KIND` (dMEV=3 / MEV=2 / PIV-outside=6 →
+# EXTRACT_OR_PIV_OUTSIDE; PIV-from-loft=5 is "as natural" → None). Left None,
+# `ventilation_from_cert` silently defaults a mechanical system to NATURAL
+# (24d), dropping its ACH heat loss (issue #1590 bug 4).
+_PASHUB_VENTILATION_TYPE_TO_KIND: Dict[str, Optional[str]] = {
+    "Natural": None,
+    "Mechanical Extract - Decentralised": "EXTRACT_OR_PIV_OUTSIDE",
+    "Mechanical Extract - Centralised": "EXTRACT_OR_PIV_OUTSIDE",
+    "Positive Input - From Outside": "EXTRACT_OR_PIV_OUTSIDE",
+    "Positive Input - From Loft (As Natural)": None,
+}
+
+
+def _pashub_mechanical_ventilation_kind(
+    ventilation_type: Optional[str],
+) -> Optional[str]:
+    """Resolve a PasHub surveyed Ventilation type label to the
+    `MechanicalVentilationKind` enum name at the mapper boundary (ADR-0015).
+    Blank stays None (natural); a non-empty label the lookup does not cover
+    strict-raises `UnmappedPasHubLabel` so the gap is fixed here, never
+    silently under-stating ventilation heat loss downstream."""
+    if not ventilation_type:
+        return None
+    if ventilation_type not in _PASHUB_VENTILATION_TYPE_TO_KIND:
+        raise UnmappedPasHubLabel("ventilation type", ventilation_type)
+    return _PASHUB_VENTILATION_TYPE_TO_KIND[ventilation_type]
+
+
 def _map_sap_ventilation(ventilation: Ventilation) -> SapVentilation:
     return SapVentilation(
         ventilation_type=ventilation.ventilation_type,
+        mechanical_ventilation_kind=_pashub_mechanical_ventilation_kind(
+            ventilation.ventilation_type
+        ),
         draught_lobby=ventilation.draught_lobby,
         pressure_test=ventilation.pressure_test,
         open_flues_count=ventilation.number_of_open_flues,