feat(modelling): Products.ashp_bundle_cost composite ASHP cost (tracer)

First slice of the per-dwelling ASHP bundle costing (ADR-0025). Products is
the rich catalogue collection over Product, owning the catalogue math: given
a typed AshpCostInputs it sums the applicable Southern Housing rate lines
(decommission + heat-pump band + fixed cylinder + full wet distribution) into
a Cost with the separate 25% ASHP contingency. Pure -- no EpcPropertyData or
calculator. Pinned exact (1e-9) against the real rate sheet. Reuse branch,
decommission variants, fallbacks, band edges and radiator clamp follow.

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

domain/modelling/products.py

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+"""Products — the rich catalogue collection over `Product` (ADR-0025).
+
+`ProductRepository` is the IO port that fetches catalogue rows; `Products` is
+the in-memory domain collection carrying the cost-composition behaviour a single
+`Product` row cannot. A simple measure prices as one row (unit cost x area); a
+composite measure — the ASHP bundle — prices by selecting and summing many
+priced line items (the Southern Housing "HEAT PUMPS" rate sheet, ECOHT01-68).
+
+This module owns the **catalogue math** only: given a typed `AshpCostInputs` it
+filters the relevant rate lines and sums them into a `Cost`. It is deliberately
+free of `EpcPropertyData` and the `Sap10Calculator` — the dwelling
+interpretation that produces the inputs (sizing, proxies, reuse detection)
+lives in the modelling layer (ADR-0025).
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from enum import Enum
+
+from domain.modelling.contingencies import contingency_rate
+from domain.modelling.recommendation import Cost
+
+_ASHP_MEASURE_TYPE = "air_source_heat_pump"
+
+# --- Southern Housing Group ASHP rates (committed constants; moved to the
+# costs file in a later slice). Each is a fully-loaded supply+install rate. ---
+
+# Decommission an existing electric-storage system, by property size band.
+_DECOMMISSION_ELECTRIC_STORAGE_SMALL = 570.0
+_DECOMMISSION_ELECTRIC_STORAGE_LARGE = 840.0
+
+# Heat-pump install (MONOBLOC, brand-neutral), by kW size band — design heat
+# loss is rounded up to the next band (ECOHT09-13).
+_PUMP_BANDS: tuple[tuple[float, float], ...] = (
+    (5.0, 9720.0),
+    (8.0, 9840.0),
+    (11.0, 10200.0),
+    (15.0, 10680.0),
+)
+_PUMP_TOP_PRICE = 11400.0
+
+# Fixed unvented hot-water cylinder (200 L) — one per install; the cylinder-size
+# spread on the sheet is £188, treated as noise (ADR-0025).
+_CYLINDER = 2382.60
+
+# Full new wet central-heating distribution, by radiator count (ECOHT40-48).
+_DISTRIBUTION_BY_RADIATORS: dict[int, float] = {
+    4: 2220.0,
+    5: 2550.0,
+    6: 3084.0,
+    7: 3618.0,
+    8: 4152.0,
+    9: 4680.0,
+    10: 5220.0,
+    11: 5754.0,
+    12: 6288.0,
+}
+_MIN_RADIATORS = 4
+_MAX_RADIATORS = 12
+
+
+class AshpExistingSystem(Enum):
+    """The dwelling's pre-retrofit heating system, as it bears on decommission
+    cost and whether a wet distribution system can be reused (ADR-0025). The
+    modelling layer maps fuel / SAP code to one of these."""
+
+    ELECTRIC_STORAGE = "electric_storage"
+    GAS = "gas"
+    OIL = "oil"
+    LPG = "lpg"
+    ELECTRIC_OTHER = "electric_other"
+    NONE = "none"
+    OTHER = "other"
+
+
+@dataclass(frozen=True)
+class AshpCostInputs:
+    """The dwelling facts the ASHP catalogue math needs — produced by the
+    modelling layer's interpretation, never read off the EPC here (ADR-0025)."""
+
+    existing_system: AshpExistingSystem
+    is_small_property: bool
+    design_heat_loss_kw: float
+    radiator_count: int
+    has_reusable_wet_system: bool
+
+
+class Products:
+    """The catalogue collection. Owns cost composition for measures whose price
+    is not a single catalogue scalar (the ASHP bundle — ADR-0025)."""
+
+    def ashp_bundle_cost(self, inputs: AshpCostInputs) -> Cost:
+        """Compose the fully-loaded ASHP bundle total for a dwelling and pair it
+        with the separate ASHP contingency rate."""
+        total: float = (
+            self._decommission(inputs)
+            + self._heat_pump(inputs.design_heat_loss_kw)
+            + _CYLINDER
+            + self._distribution(inputs)
+        )
+        return Cost(
+            total=total, contingency_rate=contingency_rate(_ASHP_MEASURE_TYPE)
+        )
+
+    def _heat_pump(self, design_heat_loss_kw: float) -> float:
+        """Price the install at the smallest band that covers the design heat
+        loss (round up); above the largest band, the top rate applies."""
+        for max_kw, price in _PUMP_BANDS:
+            if design_heat_loss_kw <= max_kw:
+                return price
+        return _PUMP_TOP_PRICE
+
+    def _decommission(self, inputs: AshpCostInputs) -> float:
+        return (
+            _DECOMMISSION_ELECTRIC_STORAGE_SMALL
+            if inputs.is_small_property
+            else _DECOMMISSION_ELECTRIC_STORAGE_LARGE
+        )
+
+    def _distribution(self, inputs: AshpCostInputs) -> float:
+        radiators: int = max(_MIN_RADIATORS, min(_MAX_RADIATORS, inputs.radiator_count))
+        return _DISTRIBUTION_BY_RADIATORS[radiators]

tests/domain/modelling/test_products.py

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+"""Behaviour of `Products.ashp_bundle_cost` — the composite, per-dwelling ASHP
+bundle cost (ADR-0025). Pure catalogue math: given a typed `AshpCostInputs` it
+selects and sums the applicable Southern Housing rate lines (decommission +
+heat pump + cylinder + distribution) into a `Cost`, carrying the separate ASHP
+contingency. No EpcPropertyData / calculator — the dwelling interpretation that
+produces the inputs lives in the modelling layer.
+
+Costs are pinned against the real Southern Housing Group rate sheet, so the
+totals are exact (delta <= 1e-9), mirroring the cascade-pin philosophy.
+"""
+
+from domain.modelling.products import (
+    AshpCostInputs,
+    AshpExistingSystem,
+    Products,
+)
+from domain.modelling.recommendation import Cost
+
+
+def test_ashp_bundle_cost_composes_an_electric_storage_full_distribution_dwelling() -> None:
+    # Arrange — a small electric-storage dwelling: no reusable wet system, so a
+    # full new wet distribution is priced. 4 kW design heat loss (smallest pump
+    # band), 7 radiators.
+    products = Products()
+    inputs = AshpCostInputs(
+        existing_system=AshpExistingSystem.ELECTRIC_STORAGE,
+        is_small_property=True,
+        design_heat_loss_kw=4.0,
+        radiator_count=7,
+        has_reusable_wet_system=False,
+    )
+
+    # Act
+    cost: Cost = products.ashp_bundle_cost(inputs)
+
+    # Assert — decommission 570 + pump 9720 + cylinder 2382.60 + distribution
+    # (7 rads) 3618 = 16290.60, with the separate 25% ASHP contingency.
+    assert abs(cost.total - 16290.60) <= 1e-9
+    assert abs(cost.contingency_rate - 0.25) <= 1e-9