UK spine: JSA claimants 2.8× the DWP caseload — legacy-benefit reporting after the UC migration

Found while measuring the first calibrated rowwise UK local candidate (#762, receipts R8–R15 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: dwp.jsa_claimants — spine 199k against DWP 71k (2.8× at initialization). Block-insensitive, so the miss is real. The related OBR JSA spend row (obr.jobseekers_allowance, 3.4× at initialization) is reachable by reweighting and is not excluded, which says the spine has too many JSA rows rather than too much JSA per row.

Likely cause: legacy-benefit reporting after the UC migration. FRS respondents still report JSA where the live claim is UC with searching-for-work conditionality (or new-style contributory JSA is conflated with the retired income-based JSA); the spine's legacy-proxies / take-up stages then keep them as JSA claimants.

Ask: reconcile the spine's JSA claimant count against the DWP JSA caseload (contributory vs income-based, the latter structurally near zero in 2025) and reclassify FRS-reported income-based JSA to UC where the respondent's circumstances say UC; keep contributory JSA as reported.

Acceptance: the row sits within 25 % at initialization on the next spine and the A16 exclusion retires.

Refs #762, #736, #665.

Start with the spine-m calibration evidence for dwp.jsa_claimants in FRS 2024-25 and compare it with the related obr.jobseekers_allowance row. Read the legacy-proxies and take-up stages implicated by the issue, along with #762's A16 exclusion and #736 tracking. Done means the JSA row is within 25% at initialization on the next spine and the exclusion can retire.