Add certified-bundle replay pipeline for recent OBR fiscal events

[MaxGhenis]

Refs #156 (track 1). Replays recent OBR-costed fiscal events on the certified bundle (populace-uk-2023, policyengine-uk 2.89.2), generalising the Autumn Budget 2025 port (#136).

What's here

• An event-parametrised pipeline: build_uk_event_registry, compute_uk_event, stage_uk_event, compare_uk_event, the Modal UK adapter modal_uk_event and adopt_uk_event.
• Registries and descriptive comparisons for Spring Budget 2023, Autumn Statement 2023, Spring Budget 2024, Autumn Budget 2024 and Spring Statement 2025.
• docs/uk_replay/RECIPE.md and YEARS.md.
• Every source head and FY cell is accounted once. Measures by class: 0 expressible, 16 partial (13 plus LHA reset, UC childcare caps and Class 2 abolition), and the rest not expressible or outside household scope. The not-expressible measures split into 84 construction_pending and 76 model or data gaps.
• 165 computed source rows. Comparisons use descriptive bins and named axes (population vintage, behavioural, baseline vintage, CY-proxies-FY, head scope). There is no pass/fail.

Review

• GPT-6.1 Sol, first review: NEEDS-FIXES.
  • B1: an inert UC health construction.
  • B2: reform dates and the fiscal-year conversion.
  • B3: staging hid inert constructions.
  • Also M1–M3 and L1.
• All fixed in 133613e. Reruns of Spring Statement 2025, Spring Budget 2023 and Autumn Statement 2023.
• Re-review: MERGE-SAFE at 133613e. 35549bf adds only ruff format and the reviewer's suggested label ("Registered construction replay complete").

Divergence causes (from the review)

Measure	Cause
Annual Allowance £60k	Model bug, policyengine-uk#2237
SDLT surcharge +2pp	Model bug, policyengine-uk#2238
UC standard allowance	Model bug, policyengine-uk#2239, plus the CPI caveat
HICBC	Fixed on main after the certified pin
Employee NICs (~1.25×)	Certified tax base 15–20% above the OBR forecast
CGT	Pooled-gains construction (upward bias), static vs behavioural
Winter Fuel Payment	Baseline vintage
Employer NICs income tax head	Wage incidence

Invariants

• Source rows and signed/absolute £ in = classified and staged out, for every event.
• Positive = gain to the Exchequer. Reversals are -(reversal − certified).
• Head effects sum to measure totals.
• Byte-identical reruns.
• AB2025 fixtures unchanged.
• An inert in-force construction blocks completion.

Tests: 176 replay and 324 extended tests (fix lane); 128 comparison, compute and registry tests after the label change.

The PR does not touch the app or the DB build, so merging publishes nothing.

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[MaxGhenis]

Independent review (GPT-6.1 Sol, read-only): NEEDS-FIXES. Full text: ~/PolicyEngine/_worktrees/replay-briefs/track1-review-out.md on the main session's machine. Summary:

Blocking

• B1. The UC health element construction (Spring Statement 2025) is inert. The uc_reform.py modifier fixes uc_LCWRA_element before the registry reform applies, but the measure is reported as computed.
• B2. Dict reforms skip the 30 April fiscal-year conversion, contrary to YEARS.md and RECIPE.md. As a result the Annual Allowance package's FY2023-24 was lost.
• B3. Staging labels identical in-force worlds as a benign note, which hid B1 and B2.

Other findings

• M1: the UC standard allowance counterfactual uses a CPI path that differs from the legislated amount's.
• M2: the "not expressible" count mixes 87 construction_pending with 76 genuine gaps.
• M3: the divergence queue lacks the engine diagnoses.
• L1: notes.

Divergence causes found

• Model bugs, now filed:
  • policyengine-uk#2237: the Annual Allowance excess is denied relief and also charged.
  • policyengine-uk#2238: SDLT counts the whole other-property stock as an additional purchase.
  • policyengine-uk#2239: the UC standard allowance omits the 2027-29 uplifts.
• HICBC: fixed on main after the certified pin.
• Employee NICs: the certified tax base is 15-20% above the OBR forecast; uk-data#537 is not the cause.
• CGT: construction scope and behaviour.
• Winter Fuel Payment: baseline vintage.
• Employer NICs income tax head: wage incidence.

A fix lane is running: 20261009-215541-replay-track1-fixes. It also adds the three measures the review found expressible (LHA reset, UC childcare cap, Class 2 abolition).

[MaxGhenis]

Merge audit: GPT-6.1 Sol review NEEDS-FIXES at 2c231b4 (B1-B3, M1-M3, L1); fixes in 133613e; re-review MERGE-SAFE at 133613e; 35549bf adds only ruff format and the reviewer's suggested status label. Checks green at 35549bf; MERGEABLE; no app or DB-build changes, so nothing is published. Squash-merged under Max's merge-on-gates rule.