Refresh admin target vintages

[MaxGhenis]

Summary

• refresh OBR targets to the March 2026 EFO vintage
• refresh the DWP benefit cap source to the November 2025 release and update the point-in-time capped-households target
• refresh DWP two-child-limit references to the April 2025 publication
• refresh the local UC-by-children split so it matches current GB-wide child-count totals instead of fixed November 2023 bucket totals

Notes

• the local UC refresh keeps the last observed November 2023 country shares, but rescales them to the latest GB-wide 2025 UC household totals by children count; that is the cleanest update available from the public exports already in the repo
• I left tax_benefit.csv alone here because the actual calibration target registry reads from targets/sources/*, not that storage file

Verification

• pytest -q policyengine_uk_data/tests/test_target_registry.py policyengine_uk_data/tests/test_uc_by_children.py
• ruff check policyengine_uk_data/targets/sources/obr.py policyengine_uk_data/targets/sources/dwp.py policyengine_uk_data/targets/sources/local_uc.py policyengine_uk_data/tests/test_target_registry.py policyengine_uk_data/tests/test_uc_by_children.py