Add capital gains basis indexation reform

[MaxGhenis]

Summary

Adds the PolicyEngine-US side of capital gains basis indexation, stacked on #8423's new basis and holding-period inputs.

This PR adds:

• IRS capital gains indexation parameters for activation, minimum holding period, acquisition-year threshold, no-loss cap, and chained-CPI transition year
• a tax-unit-level basis indexation adjustment, computed from the collapsed tax-unit representation
• person-level allocation of the tax-unit adjustment so existing person-level long_term_capital_gains consumers continue to work
• prospective, retrospective, and retrospective-losses-allowed reform helpers for benchmark comparisons
• YAML tests covering inactive baseline behavior, prospective/retrospective eligibility, no-loss capped gains, mixed-spouse collapsed tax-unit behavior, and the losses-allowed Yale benchmark axis

Notes

This intentionally computes the adjustment at the tax-unit level first. The data imputation PR stores basis and holding period at person level for compatibility, but the collapsed imputation is conceptually one representative transaction per tax unit.

The default reform helpers use a no-loss cap. A separate capital_gains_indexation_retrospective_losses_allowed helper is included for benchmarking against the public Yale runscript, which appears to leave index_kg_no_loss = 0.

Stacked on #8423. Related data issue: PolicyEngine/policyengine-us-data#1126.

Tests

• uv run ruff check policyengine_us/variables/gov/irs/tax/federal_income/capital_gains/tax_unit_long_term_capital_gains_indexation_adjustment.py policyengine_us/variables/household/income/person/capital_gains/long_term_capital_gains_indexation_adjustment.py policyengine_us/reforms/capital_gains_indexation.py policyengine_us/variables/household/income/person/capital_gains/long_term_capital_gains.py
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/contrib/capital_gains_indexation.yaml -c policyengine_us
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/irs/tax/federal_income/capital_gains policyengine_us/tests/policy/baseline/gov/irs/integration/capital_gains_losses.yaml policyengine_us/tests/variables/household/income/person/capital_gains -c policyengine_us
• uv run pytest policyengine_us/tests/test_parameter_files.py policyengine_us/tests/test_system_import.py -q