Update 2026-27 Pension Credit additional amounts

The UK EFRS RuleSpec comparator in TheAxiomFoundation/axiom-encode#457 shows PolicyEngine UK 4.11.0 / policyengine-uk 2.88.20 is using forecast-indexed 2026 Pension Credit additional amounts rather than the published 2026-27 rates encoded in rulespec-uk.\n\nObserved on the full local Enhanced FRS 2023-24 dataset for policy year 2026: PolicyEngine's carer minimum guarantee addition is about £47.95/week per carer, while RuleSpec Regulation 6 has £48.15/week. RuleSpec also encodes the severe-disability additions at £86.05 / £172.10 per week, though the current EFRS oracle has no positive severe-disability addition rows.\n\nThis is analogous to #1740 for the standard minimum guarantee rates.\n\nRepro command from the axiom-encode PR branch:\n\nbash\nuv run --with 'policyengine[uk]==4.11.0' axiom-encode uk-efrs-compare --root /Users/maxghenis/TheAxiomFoundation --sample-size 0 --dataset /Users/maxghenis/PolicyEngine/policyengine-uk-data/policyengine_uk_data/storage/enhanced_frs_2023_24.h5 --fail-on-mismatch\n

Start with the uk-efrs-compare entry point and run the provided uv command against the enhanced_frs_2023_24.h5 dataset. Trace the Pension Credit additional-amount rules, compare them with Regulation 6's published 2026-27 rates, and confirm the comparator passes without mismatches.