10 modelling_e2e properties failed with "unmapped SAP code in fuel_code: 10":
the billing layer (`sap_code_to_fuel`) had no carrier for Table-32 code 10
(dual fuel, mineral + wood) and raised rather than guess one.
SAP 10.2 treats dual fuel as its OWN fuel (its own Table-12 factors), so model
it as its own billing carrier rather than collapsing onto wood or coal:
- New `Fuel.DUAL_FUEL_MINERAL_AND_WOOD`.
- `_CODE_TO_FUEL[10]` -> that carrier.
- Fuel Rates snapshot prices it at 7.69 p/kWh — the midpoint of the COAL proxy
(7.13) and WOOD_LOGS (8.25). This mirrors SAP's own construction: Table-32
dual fuel (3.99) ~= midpoint of house coal (3.67) and wood logs (4.23).
Marked `derived` with a documented _note/_gap/_assumption (like the COAL and
HEAT_NETWORK proxies), since there is no retail blend price.
A dedicated carrier + rate (vs a one-line map to an existing carrier) keeps the
fuel identity faithful to SAP and avoids mispricing dual fuel as pure wood/coal.
Tests: code 10 -> DUAL_FUEL carrier; snapshot prices it at 7.69; grid-export
codes (36/60) still raise (the genuine no-carrier case).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Slice 1 of Bill Derivation — the reference-data foundation that later slices
price the calculator's per-end-use kWh against:
- Fuel enum (canonical billing fuels; the join key between the calculator's
SAP-code fuels and the rates snapshot). COAL + HEAT_NETWORK are members with
no national rate.
- FuelRates value object: unit_rate_p_per_kwh / standing_charge_p_per_day /
seg_export_p_per_kwh; raises UnpricedFuel on a fuel it has no rate for rather
than billing at a wrong default.
- FuelRatesRepository port (ADR-0011 Repo-reads-stored-reference-data) +
StaticFileFuelRatesRepository reading a committed JSON snapshot.
- Snapshot fuel_rates_2026_q2.json: GB national, Apr-Jun 2026 Ofgem cap
(gas/electricity) + DESNZ/NEP May 2026 (off-gas). Carries the full researched
data; the value object exposes single-rate fuels this slice. Off-peak
(day/night), house coal and heat network raise UnpricedFuel until later slices.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
