Add OBR forecast importer

[MaxGhenis]

Summary

Adds a reusable importer for OBR detailed forecast tables and a thin CLI wrapper so future yoy_growth.yaml updates can be driven from the published economy workbook instead of manual copy/paste.

What changed

• adds policyengine_uk.utils.import_obr_forecasts to extract annual forecast series from OBR economy ZIP/XLSX files
• adds scripts/import_obr_forecasts.py for direct repo usage
• updates only the forecast window for the target OBR series and refreshes the first EFO reference in each section
• preserves existing YAML values when the workbook leaves a forecast year blank
• adds unit coverage for workbook parsing, release inference, YAML updates, and sparse-year handling

Closes #1378.