Devolved administrations (modes 1+2): SFC costings, Scottish income tax outturn, Welsh rates — the UK analogue of the US state dimension

The US side's founding population is state-resolved (Urban's State of the Safety Net; state exhibits in the app). The UK side so far treats GB/UK as one geography — HBAI region cuts (#44) are the only sub-national rows, and nothing exercises PE-UK's devolved policy machinery (Scottish income tax bands, Welsh rates, Scottish social security). No open issue covers this; it is the missing mirror of #2's state dimension.

External populations (all published, aggregate, harvestable under the uk_obr/NOTES.md recipe conventions)

Mode 2 — Scottish Fiscal Commission policy costings. The SFC publishes certified costings of Scottish Government tax and social-security measures (Scottish Budget documents + its Fiscal Sustainability / Economic and Fiscal Forecasts). These are the devolved analogue of the OBR Policy Measures Database that #54/#56 just built the lane for — same registry → managed-sim → staged-comparison shape, PE reform specs against gov.hmrc.income_tax.rates.scottish_* and Scottish social-security parameters. Computability triage will be honest: some measures (ADS, LBTT, NDR) are not_expressible in PE-UK and should be catalogued as such, exactly like the 5/26 in data/uk/obr_measure_reforms.yaml.

Mode 1 — devolved baselines.

• HMRC Scottish income tax outturn (annual publication, NSND liabilities by band) vs PE-UK Scotland-filtered liabilities on the certified bundle.
• Social Security Scotland statistics (Scottish Child Payment caseload/expenditure — a benefit PE-UK models and no current external source covers).
• SFC forecast baselines (Scottish income tax, devolved social security expenditure) as baseline_moment rows, sibling to the OBR EFO adapter (#46).
• Welsh Rates of Income Tax outturn (HMRC) — small but closes the geography.

Why now

• The geography plumbing is already under review: #51's geo_masks() (country-level masks incl. Scotland) is in flight, and its reviewer flagged silent whole-UK-labeled-GB emission — devolved rows are the population that would actually exercise that path.
• PE-UK carries the Scottish parameter tree; nothing on the Scorecard currently validates it. A Scottish starter rate / SCP claim family is a cheap first row set once the #48-vs-#32 ingest adjudication (#57) lands.

Steps

• Harvest lane: SFC costings + HMRC Scottish outturn + SSS statistics + WRIT (staged per README conventions, primary sources, Wayback originals)
• Registry: data/uk/sfc_measure_reforms.yaml mirroring the OBR registry schema (verbatim SFC description, PE parameter dict or null, computability, construction)
• Mode-1 adapter: Scottish income tax by band; SCP caseload
• Compute: Scotland-masked managed sims on the certified bundle (depends on #51's per-country masks landing)
• Assumptions-registry entries (#10) for SFC methodology (behavioural adjustment, forecast baseline vintage)