refactor(billing): relocate Bill Derivation to domain/billing/ (cross-stage)

Bill / EnergyBreakdown / BillDerivation / sap_fuel were under
domain/property_baseline/ only because Baseline was built first. The Modelling
stage now needs them too, so move them (and their tests) to a neutral
domain/billing/ — Fuel/FuelRates already live in the shared domain/fuel_rates/.
Avoids a modelling -> property_baseline cross-stage import and a package name
that wrongly implies ownership (ADR-0011, ADR-0014 amendment). Pure git mv +
import rewrite across 10 files; 40 billing/baseline/repo tests pass, pyright
strict clean. CONTEXT.md Bill Derivation location updated.

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

CONTEXT.md

@@ -118,7 +118,7 @@ The process that translates an Optimised Package into cert-field changes and pro
 _Avoid_: measure overrides (rejected during ADR-0009 grill — phantom mid-layer), package applier, retrofit simulator
 
 **Bill Derivation**:
-The deterministic process that derives a Property's annual energy **bill**, composed into per-end-use sections (heating, hot water, lighting, appliances, cooking, pumps/fans, …) plus a **total**, by pricing **SAP10 Calculation**'s delivered kWh per end use at **current Fuel Rates** — each end use billed at its fuel's rate, rolled up per fuel for **standing charges** (metered fuels only — gas/electricity; oil/LPG/solid have none) minus **SEG** export credit on PV. Implemented by `BillDerivation` in `domain/property_baseline/` (deterministic, ADR-0006). Reads Fuel Rates from a committed static snapshot via `FuelRatesRepository` (no live ETL yet). **Distinct from the calculator's `total_fuel_cost_gbp`**, which is the SAP-rating notional cost at RdSAP Table 32 standardised prices (~half the real electricity price) — not what the household pays. Raises on a fuel it has no rate for (e.g. house coal, heat network). ADR-0014.
+The deterministic process that derives a Property's annual energy **bill**, composed into per-end-use sections (heating, hot water, lighting, appliances, cooking, pumps/fans, …) plus a **total**, by pricing **SAP10 Calculation**'s delivered kWh per end use at **current Fuel Rates** — each end use billed at its fuel's rate, rolled up per fuel for **standing charges** (metered fuels only — gas/electricity; oil/LPG/solid have none) minus **SEG** export credit on PV. Implemented by `BillDerivation` in `domain/billing/` (a cross-stage concern — the Baseline stage derives the current bill, the Modelling stage re-runs it on the post-package end-state for post-retrofit bills; deterministic, ADR-0006). Reads Fuel Rates from a committed static snapshot via `FuelRatesRepository` (no live ETL yet). **Distinct from the calculator's `total_fuel_cost_gbp`**, which is the SAP-rating notional cost at RdSAP Table 32 standardised prices (~half the real electricity price) — not what the household pays. Raises on a fuel it has no rate for (e.g. house coal, heat network). ADR-0014.
 _Avoid_: EPC Energy Derivation (renamed), EpcEnergyDerivationService (no "service" suffix), kWh prediction, baseline kWh, energy estimation
 
 **UCL Correction**:

domain/billing/bill.py

@@ -6,7 +6,7 @@ from enum import Enum
 from typing import Optional, TYPE_CHECKING
 
 from domain.fuel_rates.fuel import Fuel
-from domain.property_baseline.sap_fuel import sap_code_to_fuel
+from domain.billing.sap_fuel import sap_code_to_fuel
 
 if TYPE_CHECKING:
     from domain.sap10_calculator.calculator import SapResult

domain/billing/bill_derivation.py

@@ -5,7 +5,7 @@ from typing import Final
 
 from domain.fuel_rates.fuel import Fuel
 from domain.fuel_rates.fuel_rates import FuelRates
-from domain.property_baseline.bill import (
+from domain.billing.bill import (
     Bill,
     BillSection,
     BillSectionCost,

domain/property_baseline/property_baseline_performance.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 from dataclasses import dataclass
 from typing import Optional
 
-from domain.property_baseline.bill import Bill
+from domain.billing.bill import Bill
 from domain.property_baseline.performance import Performance
 from domain.property_baseline.rebaseliner import RebaselineReason
 

infrastructure/postgres/property_baseline_performance_table.py

@@ -5,7 +5,7 @@ from typing import ClassVar, Optional, cast
 from sqlmodel import Field, SQLModel
 
 from datatypes.epc.domain.epc import Epc
-from domain.property_baseline.bill import Bill, BillSection, BillSectionCost
+from domain.billing.bill import Bill, BillSection, BillSectionCost
 from domain.property_baseline.property_baseline_performance import PropertyBaselinePerformance
 from domain.property_baseline.performance import Performance
 from domain.property_baseline.rebaseliner import RebaselineReason

orchestration/property_baseline_orchestrator.py

@@ -6,8 +6,8 @@ from datatypes.epc.domain.epc_property_data import (
     EpcPropertyData,
     RenewableHeatIncentive,
 )
-from domain.property_baseline.bill import EnergyBreakdown
-from domain.property_baseline.bill_derivation import BillDerivation
+from domain.billing.bill import EnergyBreakdown
+from domain.billing.bill_derivation import BillDerivation
 from domain.property_baseline.property_baseline_performance import PropertyBaselinePerformance
 from domain.property_baseline.performance import lodged_performance
 from domain.property_baseline.rebaseliner import Rebaseliner

tests/domain/billing/test_bill_derivation.py

@@ -4,8 +4,8 @@ import pytest
 
 from domain.fuel_rates.fuel import Fuel, UnpricedFuel
 from domain.fuel_rates.fuel_rates import FuelRate, FuelRates
-from domain.property_baseline.bill import BillSection, EnergyBreakdown, EnergyLine
-from domain.property_baseline.bill_derivation import BillDerivation
+from domain.billing.bill import BillSection, EnergyBreakdown, EnergyLine
+from domain.billing.bill_derivation import BillDerivation
 
 
 def _rates() -> FuelRates:

tests/domain/billing/test_energy_breakdown.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 import pytest
 
 from domain.fuel_rates.fuel import Fuel
-from domain.property_baseline.bill import BillSection, EnergyBreakdown
+from domain.billing.bill import BillSection, EnergyBreakdown
 from domain.sap10_calculator.calculator import SapResult
 
 

tests/domain/billing/test_sap_fuel.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 import pytest
 
 from domain.fuel_rates.fuel import Fuel
-from domain.property_baseline.sap_fuel import sap_code_to_fuel
+from domain.billing.sap_fuel import sap_code_to_fuel
 from domain.sap10_calculator.exceptions import UnmappedSapCode
 
 

tests/orchestration/test_property_baseline_orchestrator.py

@@ -10,7 +10,7 @@ from datatypes.epc.domain.epc_property_data import (
     RenewableHeatIncentive,
 )
 from domain.fuel_rates.fuel import Fuel
-from domain.property_baseline.bill import BillSection
+from domain.billing.bill import BillSection
 from domain.property_baseline.property_baseline_performance import PropertyBaselinePerformance
 from domain.property_baseline.performance import Performance
 from domain.property_baseline.rebaseliner import (

tests/repositories/property_baseline/test_property_baseline_postgres_repository.py

@@ -4,7 +4,7 @@ from sqlalchemy import Engine
 from sqlmodel import Session
 
 from datatypes.epc.domain.epc import Epc
-from domain.property_baseline.bill import Bill, BillSection, BillSectionCost
+from domain.billing.bill import Bill, BillSection, BillSectionCost
 from domain.property_baseline.property_baseline_performance import PropertyBaselinePerformance
 from domain.property_baseline.performance import Performance
 from repositories.property_baseline.property_baseline_postgres_repository import (