Add Westminster council tax support

[MaxGhenis]

Summary

• Add Westminster working-age Council Tax Support parameters and variables.
• Register Westminster in the local CTR selector and simulated CTR aggregator.
• Add source-linked baseline tests for maximum support, capital cutoff, tariff income, non-dependant deductions, and UC-specific income/capital handling.

Sources

• Westminster Council Tax Support Scheme 2026-27: https://www.westminster.gov.uk/sites/default/files/media/documents/council-tax-support-scheme-for-2026-27.pdf
• Westminster income and savings guide: https://www.westminster.gov.uk/benefits/guide-housing-benefit-and-council-tax-support/income-and-savings

Tests

• uvx ruff format --check .
• uv run policyengine-core test policyengine_uk/tests/policy/baseline/gov/local_authorities/council_tax_reduction/council_tax_reduction.yaml -c policyengine_uk
• uv run ruff check policyengine_uk/variables/gov/local_authorities/council_tax_reduction policyengine_uk/variables/gov/local_authorities/westminster
• git diff --check
• uv run policyengine-core test policyengine_uk/tests/policy -c policyengine_uk
• uv run --with pytest --with pytest-cov python -m pytest policyengine_uk/tests/ --cov=policyengine_uk --cov-report=xml --maxfail=1 -v