Read the OBR's national living wage forecast (EFO Table 1.13) in import_obr_forecasts #2721

gov.economic_assumptions.yoy_growth.obr.national_living_wage (added in #2700) is kept by hand, because utils/import_obr_forecasts.py can't read its source. This issue is to teach the importer to read it, so the series refreshes with each EFO like the others.

The source. The EFO detailed forecast tables (economy), sheet 1.13 "National Living Wage", £ per hour:

• The years run across one header row (2020 to 2031 in October 2026), with the levels in the row below.
• A note says the rate "has been set by the Government up until 2027" and that later figures "are OBR estimates".

What the importer reads today. Every SERIES_SPECS entry is a growth rate read down a column, keyed by a row label (SeriesSpec.column and needles). Table 1.13 has neither shape: it is levels, laid out across a row.

What the series needs.

• Growth: each year's level over the year before's, to six decimals, from the table's second year to its last. Compounding them reproduces the table to the penny.
• 2032 onwards: average_earnings' long-run path, year for year, as for the receipts series (average_earnings_path).
• Comments and references: the YAML block's comment and references updated to the release.

Done when:

• The importer writes obr.national_living_wage from the workbook, with a test on a fixture workbook in test_import_obr_forecasts.py.
• test_minimum_wage_projection.py's OBR_NATIONAL_LIVING_WAGE table is refreshed in the same run, or read from the workbook fixture.
• The README's "updated by hand" note and the YAML's "Refresh 2028 onwards with each EFO" go.

Related:

• Update the OBR forecasts to the October 2026 Economic and fiscal outlook #2711 (October 2026 EFO update): if it lands first, copy its new average_earnings 2032+ values into this series. test_after_the_efo_the_series_is_average_earnings_growth fails until someone does.
• Project the minimum wage rates by the OBR's national living wage forecast #2700: why this series exists. HM Treasury costed the Budget 2026 Carer's Allowance taper on the OBR's national living wage projections.