Add Housing Benefit capital rules and tariff income

[MaxGhenis]

Closes #1576

Summary

• add legislation-backed Housing Benefit capital parameters for working-age and pension-age cases
• add assessable-capital and tariff-income variables to the Housing Benefit means test
• apply the upper-capital-limit eligibility rule and Guarantee Credit passporting
• add YAML regression coverage for thresholds, tariff-income rounding, pension-age treatment, and shared-household allocation

Testing

• uvx ruff check policyengine_uk/variables/gov/dwp/housing_benefit policyengine_uk/tests/policy/baseline/finance/benefit/family/housing_benefit policyengine_uk/parameters/gov/dwp/housing_benefit/means_test/capital
• uv run policyengine-core test policyengine_uk/tests/policy/baseline/finance/benefit/family/housing_benefit/housing_benefit_capital.yaml policyengine_uk/tests/policy/baseline/finance/benefit/family/housing_benefit/housing_benefit_eligible.yaml policyengine_uk/tests/policy/baseline/finance/benefit/family/housing_benefit/applicable_income/housing_benefit_applicable_income.yaml policyengine_uk/tests/policy/baseline/finance/benefit/family/housing_benefit/entitlement/housing_benefit_entitlement.yaml policyengine_uk/tests/policy/baseline/finance/benefit/family/housing_benefit/housing_benefit.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:

• Housing Benefit spending: £14.54bn -> £12.07bn (-17.0%)
• Housing Benefit recipient benunits: 1.675m -> 1.247m (-25.6%)

For context, when this is combined with the current Income Support and Pension Credit capital-rule PRs as well, Housing Benefit ends up at £11.47bn (-21.1% vs current main) and 1.159m recipients (-30.8%). The extra drop there is the overlap/spillover from the other legacy means tests, not just the direct HB capital test.