Add Pension Credit capital rules

[MaxGhenis]

Closes #1574

Summary

• add legislation-backed Pension Credit capital parameters and a capital-source list parameter
• replace direct capital-income counting with deemed income from capital above the threshold
• add a household-capital proxy that pools capital across pension-age adults without diluting by unrelated working-age adults
• add YAML regressions for threshold breakpoints, mixed-age households, separate pension-age benunits, and Guarantee Credit interaction

Testing

• /opt/homebrew/bin/ruff check policyengine_uk/variables/gov/dwp/pension_credit policyengine_uk/parameters/gov/dwp/pension_credit policyengine_uk/tests/policy/baseline/gov/dwp/pension_credit
• ./.venv/bin/policyengine-core test policyengine_uk/tests/policy/baseline/gov/dwp/pension_credit/is_pension_credit_eligible.yaml policyengine_uk/tests/policy/baseline/gov/dwp/pension_credit/pension_credit_income.yaml policyengine_uk/tests/policy/baseline/gov/dwp/pension_credit/pension_credit_deemed_income.yaml policyengine_uk/tests/policy/baseline/gov/dwp/pension_credit/pension_credit_earnings.yaml policyengine_uk/tests/policy/baseline/gov/dwp/pension_credit/guarantee_credit/is_guarantee_credit_eligible.yaml policyengine_uk/tests/policy/baseline/gov/dwp/pension_credit/savings_credit/meets_savings_credit_age_requirement.yaml policyengine_uk/tests/policy/baseline/gov/dwp/pension_credit/savings_credit/is_savings_credit_eligible.yaml policyengine_uk/tests/policy/baseline/gov/dwp/pension_credit/housing_benefit.yaml -c policyengine_uk

[MaxGhenis]

Aggregate effect on the current calibrated enhanced_frs_2023_24 dataset at 2026, measured on a clean main + this PR's files code state rather than the raw branch tip:

• Pension Credit spending: £2.63bn -> £1.49bn (-43.3%)
• Pension Credit recipient benunits: 692.7k -> 327.0k (-52.8%)

I did not see any material spillover onto Housing Benefit or Income Support from this PR in the isolated clean-state run.