Fix stock capital period handling

[MaxGhenis]

Summary

• Mark UK capital stock variables as STOCK so monthly calculations do not prorate balances.
• Preserve stock values when aggregating household capital sources into benefit-unit capital tests for UC, Pension Credit, and Housing Benefit.
• Add regression tests showing monthly UC and Pension Credit capital calculations keep savings/property/corporate capital as stocks.

Tests

• uv run pytest policyengine_uk/tests/test_stock_capital_periods.py
• uv run policyengine-core test policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/is_UC_eligible.yaml policyengine_uk/tests/policy/baseline/gov/dwp/pension_credit/pension_credit_deemed_income.yaml policyengine_uk/tests/policy/baseline/finance/benefit/family/income_support/income_support_capital.yaml policyengine_uk/tests/policy/baseline/finance/benefit/family/jsa_income/jsa_income_capital.yaml -c policyengine_uk
• uv run policyengine-core test policyengine_uk/tests/policy -c policyengine_uk
• uv run pytest policyengine_uk/tests/ --maxfail=1 -q
• uv run ruff check policyengine_uk/variables/household/wealth/savings.py policyengine_uk/variables/household/wealth/net_financial_wealth.py policyengine_uk/variables/household/wealth/gross_financial_wealth.py policyengine_uk/variables/household/wealth/total_wealth.py policyengine_uk/variables/gov/hmrc/student_loans/student_loan_balance.py policyengine_uk/variables/gov/dwp/universal_credit/uc_assessable_capital.py policyengine_uk/variables/gov/dwp/universal_credit/uc_reported_capital.py policyengine_uk/variables/gov/dwp/universal_credit/household_uc_reported_capital.py policyengine_uk/variables/gov/dwp/housing_benefit/housing_benefit_assessable_capital.py policyengine_uk/variables/gov/dwp/pension_credit/pension_credit_assessable_capital.py policyengine_uk/variables/gov/dwp/esa_income_assessable_capital.py policyengine_uk/variables/gov/dwp/jsa_income_assessable_capital.py policyengine_uk/variables/gov/dwp/income_support_assessable_capital.py policyengine_uk/tests/test_stock_capital_periods.py