fix(elmhurst): map secondary room-heater SAP codes to Table 4a fuel category

Completes `_elmhurst_secondary_fuel_from_sap_code` per SAP 10.2 §12
(PDF p.34: "Secondary heating systems and applicable fuel types are taken
from the room heaters section of Table 4a") + RdSAP 10 §10.4.1. Each
Table 4a room-heater code now resolves to its fuel CATEGORY's modal fuel:

  - gas room heaters    601-613 → mains gas   (26 → Table 32 1, 3.48 p/kWh)
  - liquid room heaters 621-625 → heating oil (28 → Table 32 4, 5.44 p/kWh)
  - solid room heaters  631-636 → house coal  (11 → Table 32 11, 3.67 p/kWh)
  - electric room htrs   691-694/699/701 → None (cascade electricity default)

Previously only the gas (601-613→26) and solid (631-634→11) blocks were
mapped; liquid heaters (621-625) and 635-636 fell through to None →
silently billed as electricity (13.19 p/kWh), a large mis-price for an
oil/solid heater. The prior slice raised on those; this maps them to the
correct category fuel instead, and keeps the raise ONLY for codes inside
the room-heater range (601-701) that are not a recognised Table 4a row.

The specific sub-fuel within a category (mains gas vs LPG vs biogas) is a
SEPARATE lodgement per §10.4.1 and is NOT exported in the Summary, so the
gas block stays the modal mains gas — worksheet "simulated case 37" lodged
its 605 live-effect fire on biogas (7.60 p/kWh), unrecoverable from the
Summary code alone (this is the entire +7 SAP case-37 gap: secondary
energy £131 + a separate biogas standing charge £70; every other line
matches the worksheet exactly, incl. (206) main efficiency 61%).

5 AAA tests, harness 47/47 (0 raised), pyright net-zero, regression clean,
corpus gauge unchanged (Elmhurst-path only).

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

datatypes/epc/domain/mapper.py

@@ -5332,6 +5332,35 @@ def _elmhurst_pump_age_int(age_str: Optional[str]) -> Optional[int]:
     return 2
 
 
+# SAP 10.2 Table 4a "Room heaters" section — the secondary-heating SAP
+# code → FUEL CATEGORY map. Per RdSAP 10 §10.4.1 + SAP 10.2 §12 (PDF
+# p.34, "Secondary heating systems and applicable fuel types are taken
+# from the room heaters section of Table 4a") each code carries an
+# appliance type whose APPLICABLE FUELS are a category (gas / liquid /
+# solid / electric); the SPECIFIC sub-fuel within that category (mains
+# gas vs LPG vs biogas) is lodged SEPARATELY and is NOT recoverable from
+# the Summary, which exports only the code. We therefore resolve each
+# code to its category's MODAL fuel. Codes mirror the Table 4a efficiency
+# rows in `domain.sap10_ml.sap_efficiencies` (601-613 gas, 621-625
+# liquid, 631-636 solid, 691-694/699/701 electric).
+_ELMHURST_SECONDARY_GAS_CODES: Final[frozenset[int]] = frozenset(
+    {601, 602, 603, 604, 605, 606, 607, 609, 610, 611, 612, 613}
+)
+_ELMHURST_SECONDARY_LIQUID_CODES: Final[frozenset[int]] = frozenset(
+    {621, 622, 623, 624, 625}
+)
+_ELMHURST_SECONDARY_SOLID_CODES: Final[frozenset[int]] = frozenset(
+    {631, 632, 633, 634, 635, 636}
+)
+_ELMHURST_SECONDARY_ELECTRIC_CODES: Final[frozenset[int]] = frozenset(
+    {691, 692, 693, 694, 699, 701}
+)
+# Modal Table 32 / `_ELMHURST_MAIN_FUEL_TO_SAP10` fuel code per category.
+_SECONDARY_FUEL_MAINS_GAS: Final[int] = 26  # Table 32 code 1, 3.48 p/kWh
+_SECONDARY_FUEL_HEATING_OIL: Final[int] = 28  # → Table 32 code 4, 5.44 p/kWh
+_SECONDARY_FUEL_HOUSE_COAL: Final[int] = 11  # Table 32 code 11, 3.67 p/kWh
+
+
 def _elmhurst_secondary_fuel_from_sap_code(
     sap_code: Optional[int],
 ) -> Optional[int]:
@@ -5342,49 +5371,43 @@ def _elmhurst_secondary_fuel_from_sap_code(
     electricity when `secondary_fuel_type` is None — correct for the
     portable-electric default but wrong for fuel-fired room heaters.
 
-    SAP 10.2 Table 4a Category 10 ("Room heaters") code blocks:
-      601-613: Gas (mains gas / LPG / biogas) — column A is mains gas;
-               column B for LPG. Cohort default is mains gas
-               (`_ELMHURST_MAIN_FUEL_TO_SAP10["Mains gas"] = 26`).
-      621-625: Liquid fuel room heaters (oil / bioethanol). Cohort
-               not yet exercised — NOT mapped, so they RAISE (see below).
-      631-634: Solid fuel room heaters (open fire, closed room
-               heater with/without boiler). House coal is the modal
-               default per Table 12 secondary rate (3.67 p/kWh).
-      691-699: Electric room heaters. Cascade default (None) routes
-               to standard electricity (13.19 p/kWh).
+    Each code resolves to its SAP 10.2 Table 4a fuel CATEGORY's modal
+    fuel (per RdSAP 10 §10.4.1 — the specific sub-fuel is a separate
+    lodgement the Summary omits):
+      - gas room heaters (601-613)      → mains gas (26)
+      - liquid room heaters (621-625)   → heating oil (28)
+      - solid room heaters (631-636)    → house coal (11)
+      - electric room heaters (691-699, → None (cascade electricity
+        701)                              default; electricity IS the fuel)
+
+    A code in the room-heater range (601-701) that is NOT a recognised
+    Table 4a row RAISES `UnmappedElmhurstLabel` rather than silently
+    mis-fuelling — per the strict-raise pattern. Codes outside the range
+    (e.g. None / no secondary) return None.
 
     Cohort cert 2102-3018-0205-7886-5204 surfaces the 631 ("Open fire
-    in grate") path — pre-slice the cascade defaulted to electricity
-    at 13.19 p/kWh, over-charging secondary by ~£340/yr and pushing
-    SAP -15.81 below the worksheet's 63.87.
+    in grate") path — pre-fix the cascade defaulted to electricity at
+    13.19 p/kWh, over-charging secondary by ~£340/yr and pushing SAP
+    -15.81 below the worksheet's 63.87.
 
-    RAISE-don't-guess: a Category-10 room-heater code (601-699) that is
-    fuel-fired but NOT in the mapped gas/solid blocks (e.g. 621-625 liquid
-    fuel) used to return None → the cascade silently billed it as
-    electricity (13.19 p/kWh), a large mis-price for an oil/LPG heater.
-    Per the `UnmappedElmhurstLabel` strict-raise pattern these now raise
-    so the gap surfaces instead of producing a wrong SAP. Electric room
-    heaters (691-699) keep returning None — electricity IS their fuel.
-
-    NOTE the gas block 601-613 still resolves to the MODAL default mains
-    gas: the Summary lodges only the SAP code, never the gas sub-fuel, so
-    an LPG or biogas live-effect fire (worksheet "simulated case 37" used
-    biogas at 7.60 p/kWh) is indistinguishable here from mains gas — the
-    cohort default is correct for the common case and the rarer sub-fuels
-    are not recoverable from the Summary export.
+    SUB-FUEL CAVEAT: the gas block 601-613 resolves to the modal mains
+    gas; an LPG or biogas live-effect fire (worksheet "simulated case 37"
+    lodged biogas at 7.60 p/kWh vs mains gas 3.48 p/kWh) is
+    indistinguishable here — the sub-fuel is not in the Summary export.
     """
     if sap_code is None:
         return None
-    if 601 <= sap_code <= 613:
-        return 26  # Mains gas, matching `_ELMHURST_MAIN_FUEL_TO_SAP10`
-    if 631 <= sap_code <= 634:
-        return 11  # House coal (Coal in `_ELMHURST_MAIN_FUEL_TO_SAP10`)
-    if 691 <= sap_code <= 699:
+    if sap_code in _ELMHURST_SECONDARY_GAS_CODES:
+        return _SECONDARY_FUEL_MAINS_GAS
+    if sap_code in _ELMHURST_SECONDARY_LIQUID_CODES:
+        return _SECONDARY_FUEL_HEATING_OIL
+    if sap_code in _ELMHURST_SECONDARY_SOLID_CODES:
+        return _SECONDARY_FUEL_HOUSE_COAL
+    if sap_code in _ELMHURST_SECONDARY_ELECTRIC_CODES:
         return None  # Electric room heaters → cascade electricity default
-    if 601 <= sap_code <= 699:
-        # Fuel-fired Category-10 room heater we do not yet map — raise
-        # rather than let the cascade silently bill it as electricity.
+    if 601 <= sap_code <= 701:
+        # A room-heater-range code we don't recognise — raise rather than
+        # let the cascade silently bill it as electricity.
         raise UnmappedElmhurstLabel("secondary_heating.sap_code", str(sap_code))
     return None
 

datatypes/epc/domain/tests/test_from_site_notes.py

@@ -750,10 +750,36 @@ class TestElmhurstSecondaryFuelFromSapCode:
         # Assert
         assert fuel is None
 
-    def test_unmapped_liquid_fuel_room_heater_raises(self) -> None:
-        # Arrange — SAP code 621 (liquid-fuel room heater) is fuel-fired but
-        # not in the mapped gas/solid blocks; returning None would silently
-        # bill it as electricity. It must raise instead.
+    def test_liquid_fuel_room_heater_resolves_to_heating_oil(self) -> None:
+        # Arrange — SAP code 621 (liquid-fuel room heater) → its Table 4a
+        # category's modal fuel, heating oil (28 → Table 32 code 4), NOT a
+        # silent electricity fallback.
+        from datatypes.epc.domain.mapper import (
+            _elmhurst_secondary_fuel_from_sap_code,  # pyright: ignore[reportPrivateUsage]
+        )
+
+        # Act
+        fuel = _elmhurst_secondary_fuel_from_sap_code(621)
+
+        # Assert
+        assert fuel == 28
+
+    def test_solid_fuel_room_heater_resolves_to_house_coal(self) -> None:
+        # Arrange — SAP code 631 (open fire in grate) → house coal (11).
+        from datatypes.epc.domain.mapper import (
+            _elmhurst_secondary_fuel_from_sap_code,  # pyright: ignore[reportPrivateUsage]
+        )
+
+        # Act
+        fuel = _elmhurst_secondary_fuel_from_sap_code(631)
+
+        # Assert
+        assert fuel == 11
+
+    def test_unrecognised_room_heater_range_code_raises(self) -> None:
+        # Arrange — SAP code 620 sits in the room-heater range (601-701) but
+        # is not a recognised Table 4a row; rather than silently mis-fuel it
+        # must raise.
         from datatypes.epc.domain.mapper import (
             UnmappedElmhurstLabel,
             _elmhurst_secondary_fuel_from_sap_code,  # pyright: ignore[reportPrivateUsage]
@@ -761,4 +787,4 @@ class TestElmhurstSecondaryFuelFromSapCode:
 
         # Act / Assert
         with pytest.raises(UnmappedElmhurstLabel):
-            _elmhurst_secondary_fuel_from_sap_code(621)
+            _elmhurst_secondary_fuel_from_sap_code(620)