feat(modelling): ASHP decommission fallbacks for off-sheet systems

Slice 4 of ADR-0025 costing. ASHP is offered to any house regardless of fuel,
so _decommission now prices a fallback instead of raising: no system -> 0,
electric room/panel heaters -> electric-storage line, anything else -> gas
line (representative default). Never blocks ASHP eligibility.

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

domain/modelling/products.py

@@ -136,7 +136,15 @@ class Products:
             return _DECOMMISSION_OIL
         if inputs.existing_system is AshpExistingSystem.LPG:
             return _DECOMMISSION_LPG
-        raise ValueError(f"no decommission rate for {inputs.existing_system}")
+        # Systems off the rate sheet: ASHP is still offered (ADR-0025), so price
+        # a fallback rather than raise. Nothing to remove for no system; electric
+        # room/panel heaters are comparable work to storage heaters; anything
+        # else takes the gas wet-system line as a representative default.
+        if inputs.existing_system is AshpExistingSystem.NONE:
+            return 0.0
+        if inputs.existing_system is AshpExistingSystem.ELECTRIC_OTHER:
+            return _DECOMMISSION_ELECTRIC_STORAGE_SMALL if inputs.is_small_property else _DECOMMISSION_ELECTRIC_STORAGE_LARGE
+        return _DECOMMISSION_GAS
 
     def _distribution(self, inputs: AshpCostInputs) -> float:
         radiators: int = max(_MIN_RADIATORS, min(_MAX_RADIATORS, inputs.radiator_count))

tests/domain/modelling/test_products.py

@@ -84,3 +84,30 @@ def test_reusable_wet_system_prices_a_flush_plus_half_the_distribution() -> None
     # Assert — decommission 720 + pump 9840 + cylinder 2382.60 + distribution
     # (flush 168 + 0.5 x 4152 = 2244) = 15186.60.
     assert abs(cost.total - 15186.60) <= 1e-9
+
+
+def _small_no_reuse(system: AshpExistingSystem) -> AshpCostInputs:
+    """A small dwelling, 4 kW band, 7 radiators, no reusable wet system — pump
+    9720 + cylinder 2382.60 + distribution (7) 3618 = 15720.60 common base."""
+    return AshpCostInputs(
+        existing_system=system,
+        is_small_property=True,
+        design_heat_loss_kw=4.0,
+        radiator_count=7,
+        has_reusable_wet_system=False,
+    )
+
+
+def test_decommission_falls_back_for_systems_not_on_the_rate_sheet() -> None:
+    # Arrange — the rate sheet covers gas/oil/LPG/electric-storage, but ASHP is
+    # offered to any house regardless of fuel (ADR-0025): no system costs nothing
+    # to remove; electric room/panel heaters use the electric-storage line; any
+    # other system defaults to the gas line — never a raise (that would wrongly
+    # block ASHP eligibility).
+    products = Products()
+    base = 15720.60
+
+    # Act / Assert
+    assert abs(products.ashp_bundle_cost(_small_no_reuse(AshpExistingSystem.NONE)).total - (base + 0.0)) <= 1e-9
+    assert abs(products.ashp_bundle_cost(_small_no_reuse(AshpExistingSystem.ELECTRIC_OTHER)).total - (base + 570.0)) <= 1e-9
+    assert abs(products.ashp_bundle_cost(_small_no_reuse(AshpExistingSystem.OTHER)).total - (base + 720.0)) <= 1e-9