The overlay logs an error when it cannot complete a heating companion set 🟩

A system-replacing override whose category or control is not yet mapped
now logs an error naming the archetype and code, so the incomplete
companion set (which otherwise silently inherits the replaced system's
value) is visible for review — log-and-continue, matching flag_fuel_mismatch.

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

domain/epc/property_overlays/main_heating_system_overlay.py

@@ -28,6 +28,7 @@ are left UNKNOWN until modelled. Unresolvable values produce no overlay.
 
 from __future__ import annotations
 
+import logging
 from typing import Optional
 
 from domain.modelling.simulation import EpcSimulation, HeatingOverlay
@@ -35,6 +36,8 @@ from domain.sap10_calculator.tables.table_12a import (
     OFF_PEAK_IMPLYING_HEATING_CODES,
 )
 
+logger = logging.getLogger(__name__)
+
 # Off-peak (Economy 7) meter. Electric storage / CPSU systems charge overnight at
 # the low rate and cannot run economically on a single-rate meter; "Dual" lets
 # the §12 dispatch resolve the specific tariff (storage 7-hour, CPSU 10-hour).
@@ -351,13 +354,32 @@ def main_heating_overlay_for(
         return None
     if code in _GAS_BOILER_CODES:
         return EpcSimulation(heating=_gas_boiler_overlay(code))
+    category = _category_for(code)
+    control = _control_for(code)
+    if category is None or control is None:
+        # A system-replacing override should stamp a complete, coherent companion
+        # set; an unmapped category or control leaves the field unset, so the
+        # effective cert inherits the REPLACED system's value (a silent mis-rating
+        # — e.g. a room heater keeping a storage heater's category/control). We do
+        # not yet have confident defaults for every archetype, so log-and-continue
+        # (matching `flag_fuel_mismatch`) to make the gap visible for review
+        # rather than raise mid-run.
+        logger.error(
+            "Landlord main_heating_system %r (SAP code %d) overlaid with an "
+            "incomplete companion set (category=%s, control=%s); the effective "
+            "cert may inherit the replaced system's value — known gap, review",
+            main_heating_value,
+            code,
+            category,
+            control,
+        )
     return EpcSimulation(
         heating=HeatingOverlay(
             sap_main_heating_code=code,
-            main_heating_category=_category_for(code),
+            main_heating_category=category,
             main_fuel_type=_natural_fuel_for(code),
             meter_type=_meter_for(code),
-            main_heating_control=_control_for(code),
+            main_heating_control=control,
             # A landlord override describes the existing dwelling, so its assumed
             # off-peak meter must not downgrade a more-off-peak cert meter
             # (e.g. a 24-hour all-low tariff). Measures, which re-meter for real,