Update 2026-27 benefit and tax parameters

[MaxGhenis]

Summary

• Update published 2026-27 UK benefit parameters, including Child Benefit, Pension Credit, Universal Credit, disability benefits, Housing Benefit allowances, and legacy benefit rates.
• Update 2026-27 tax-facing parameters, including minimum wage, National Insurance thresholds, student loan thresholds, and dividend tax thresholds.
• Preserve UC rebalancing behavior by treating published 2026-27 standard allowances as final amounts, while keeping existing-claimant LCWRA protection separate from the baseline LCWRA rate.
• Add regressions for CGT, Child Benefit, Pension Credit, UC LCWRA/standard allowance, minimum wage, NI Class 4, student loan thresholds, and dividend tax thresholds.

Fixes #1687
Fixes #1688
Fixes #1689

Tests

• git diff --check
• uv run policyengine-core test policyengine_uk/tests/policy -c policyengine_uk (1035 passed)
• uv run pytest policyengine_uk/tests/test_uc_rebalancing.py (4 passed, existing pydantic deprecation warning)
• /cycle review loop completed with no actionable findings in the final pass