UK static aging: ONS 2024-based projection reader and weights-only annual projections to 2050-51 #1075

Part of PolicyEngine/microcosm-dynamics#501 (WP6). The plan is at docs/plans/uk-state-pension-dynamics.md (merged in PolicyEngine/microcosm-dynamics#502). This is the "Multi-year runs, to be a separate piece" that #1069 defers.

Why

docs/static-aging.md says "the package does not yet include a UK projection reader". The UK State Pension projection needs per-year household weights that carry demographics only (#333): weighted persons by single age, sex and nation that follow the ONS 2024-based national population projections, with the base year unchanged.

Scope

• Reader. A reader for the ONS 2024-based projections: the Population sheet, single ages 0-104, 105-109 and 110+, by sex, 2024-2124, for the UK, GB and each nation. Principal projection plus the zero-net-migration and life-expectancy variants. It reads from the Chronicle package (WP5), as the UK target invariants in UK dataset build: full migration from policyengine-uk-data (master epic) #665 require; a sha256-pinned copy serves only unscored first light.
• Fiscal-year growth. 0.75 × mid-Y + 0.25 × mid-(Y+1), declared as a Microcosm-side computed target. PolicyEngine UK reads age at 6 October under Set State Pension age from date of birth, including the rise to 67 policyengine-uk#1899.
• Cells. Single ages 60-89 by sex for GB, 90+ pooled, five-year bands below 60, and Northern Ireland separately.
• Age-cell column. A build-written column, because static aging matches cells by exact equality and the UK frame stores drawn ages 80-97 for the top-coded tail.
• UK annual builder. It maps no monetary series, a deliberate departure from the factor step: factors would fix one macro path into every consumer job. Years run from the base to 2050-51. The scorecard reports each input that is uprated by a national total against its index.
• Anchoring. anchor="frame", so the base-year weights equal the base weights. A weight-ratio cap. Effective sample size by age published in every run.

Acceptance

• Every cell meets its anchored target (the base's weighted count × the ONS fiscal-year-blended growth) within the calibrator's reported tolerance. Any cell where the weight-ratio cap binds is listed.
• ONS levels are reported beside the targets as diagnostics. Examples: GB 66+ mid-2024 12,061,579; GB 67+ mid-2039 14,819,804.
• The base year's weights equal the base dataset's.
• A byte-identical rebuild from the same inputs.
• No program count or amount is a target.

Depends on

• The UK pensions: calibrate State Pension, Pension Credit and pension contributions to the resident administrative facts (PolicyEngine/chronicle#305), pin the State Pension age boundary, fix the pension spine stages, and the engine pieces a State Pension uprating analysis needs #1069 base (or an interim base by ruling).
• The ONS 2024-based projection package (WP5).

Estimate: 2-3 agent-days; 0.5-1 human review day.