UK spine: savings interest income a third under HMRC SPI and five times under the ONS D.41 row — reconcile the target concept and review FRS interest capture

[juaristi22]

Found while measuring the first calibrated rowwise UK local candidate (#762, receipts R8–R15 in experiments/762-uk-rowwise-candidate-receipts.md on uk-rowwise-candidate-762-b). The row is a reviewed measure exclusion under #762 A16, expiring 2026-10-03, tracked on #736.

Measured on spine-m (FRS 2024-25, policyengine-uk 2.92.1, calibration year 2025), single-block engine resolution: the spine carries £13.8bn of savings_interest_income at 2025 (the FRS input, engine-uprated on the ONS household-interest index). Three published figures sit under the same name:

figure	value	concept
spine (FRS-based)	£13.8bn	cash interest households report to the FRS, uprated to 2025
HMRC SPI 2023-24 (Ledger hmrc.spi_savings_interest_income_amount, sum over the income bands)	£18.3bn	taxable interest of taxpayers; ISA interest excluded
ONS UKEA HAXV (Ledger ons.savings_interest_income, the calibration target)	£95.5bn for 2025 (£19.6bn in 2021, £86.0bn in 2023)	households' (S.14) D.41 interest resources in the national accounts: FISIM-adjusted, includes the unincorporated-business part of the household sector

So the gap has two parts: a frame gap of roughly a third against the HMRC taxpayer figure (larger once 2023-24 is uprated to 2025), and a concept gap of four- to five-fold between cash interest and national-accounts D.41 resources. Bound as-is, the row is unreachable within the doctrine's stretch bound and, under grain_equal, pulls the joint solve toward weight concentration.

Ask (target side and frame side):

1. A declared reconciliation for the target: bind the HMRC SPI savings-interest amount (taxpayer concept, uprated on HAXV), or declare a D.41-to-cash bridge for the ONS row, per the "translate published facts, never drop" doctrine — the chronicle-side question is which concept the contract means.
2. A review of FRS interest capture in the spine (an imputation from WAS financial wealth × deposit rates, or an SPI-band-consistent scaling) so the frame reaches the taxpayer figure.

Acceptance: on the next spine the row (under whichever concept is declared) sits within 25 % at initialization and the A16 exclusion retires (a stale exclusion fails the run).

Refs #762, #736, #665.