OBR importer does not refresh the receipts growth series that project corporate SDLT and business rates #2341

#2322 adds four OBR receipts growth series under gov.economic_assumptions.yoy_growth.obr:

• stamp_duty_land_tax.residential and .non_residential (EFO receipts Table 3.7);
• business_rates.great_britain (current receipts on a cash basis: Table 3.8 in March 2026, 3.9 in March 2025);
• business_rates.northern_ireland (Table 3.2).

They project the corporate SDLT and business rates statistics from their latest outturn. They were derived by hand from the March 2026 EFO. policyengine_uk/utils/import_obr_forecasts.py does not refresh them: read_calendar_values skips nested series and replace_series_section handles only top-level ones. At the next EFO they would stay on March 2026, the same way the old statistics were left at 2018-2021 values.

The importer should rewrite them from a new EFO's receipts tables:

• each value is the ratio of consecutive published levels minus 1, to six decimals;
• the outturn year's growth is its outturn over the previous year's outturn in an earlier EFO (March 2025 for March 2026);
• years after the OBR's receipts forecast follow average_earnings, as test_corporate_receipts_statistics.py requires;
• tables are found by title, since their numbers move between EFOs.

Running it on the March 2026 tables should reproduce the committed values exactly.