Update 2026-27 Universal Credit Regulation 36 amounts

The new 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 Universal Credit Regulation 36 amounts rather than the published 2026-27 table amounts encoded in rulespec-uk.

Observed on the full local Enhanced FRS 2023-24 dataset for policy year 2026:

• UC standard allowance PE monthly values are about 335.30 / 423.26 / 526.30 / 664.39, but RuleSpec Regulation 36 has 338.58 / 424.90 / 528.34 / 666.97.
• UC child element PE monthly values are about 350.53 and 302.77, but RuleSpec has 351.88 and 303.94.
• UC disabled child additions PE monthly values are about 164.16 and 512.73, but RuleSpec has 164.79 and 514.71.
• UC LCWRA/carer/childcare component values show the same pattern for 2026-27 published rates versus forecast/uprated PE values.

This is analogous to the Child Benefit and Pension Credit 2026-27 rate issues found by the same harness: #1739 and #1740.

Repro command from the axiom-encode PR branch:

uv 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

Start with the uk-efrs-compare comparator and run the supplied uv command against the Enhanced FRS dataset to reproduce the 2026 mismatches. Trace how Universal Credit Regulation 36 amounts are sourced for 2026-27, then verify that standard allowances, child elements, disabled-child additions, LCWRA, carer, and childcare values match the published table rather than forecasts.