Refresh ons.population and re-derive the per-capita series from one vintage #2383

gov.economic_assumptions.yoy_growth.ons.population is a March 2025 vintage, while the per-capita series built on it use March 2026 numerators.

Where it comes from

Every value from 2022 to 2073 is the "Population growth" row of the OBR's Long-term economic determinants – March 2025 EFO (published 19 June 2025), brought in by #1305. The fiscal year starting in year t is stored at t-01-01, rounded to four decimals. Footnote 2 of that table: through 2029-30 it is the OBR's March 2025 forecast, and after that the migration category variant of the ONS 2022-based national population projections. The 2021 value (0.004) predates #1305 and has no recorded source.

Why it is stale

• Calendar-year population growth implied by the March 2026 EFO economy tables (Table 1.1 real GDP over Table 1.5 real GDP per person) is 0.37% for 2025 against 0.72% stored, and 0.30–0.36% a year for 2026–2030 against 0.37–0.45%.
• The ONS published 2024-based national population projections on 28 April 2026, with lower migration and fertility assumptions than the 2022-based ones.
• per_capita.gdp and per_capita.mixed_income are March 2026 nominal growth minus this series, so they mix vintages.

Size

Swapping in the March 2026 implied population while keeping the same numerators would raise the per-capita GDP and mixed income factors from 2024 by about 0.35% by 2025, 0.46% by 2027 and 0.73% by 2030. household_weight is uprated by the same series, so in the uprating step aggregates move with nominal growth either way; the vintage mostly shifts the split between amounts per household and the number of households.

To do

• Decide the target: the March 2026 EFO's population path to 2030-31 and the ONS 2024-based projections after, or the next OBR long-term determinants release if one is published for the March 2026 EFO.
• Re-derive ons.population and every per_capita.* series from it together, and cite the dated release in each series' metadata.
• Record the 2021 source, or drop the value if it cannot be found.

Context: #1863, which documents the current provenance in the YAML.