Fix UC rebalancing protection for existing claimants

[MaxGhenis]

Summary

• protect existing UC health-element claimants by keeping the combined standard allowance and health element CPI-protected through 2029-30
• add the extra single-under-25 standard allowance top-up required by the November 2025 rebalancing design
• cover the existing-claimant, under-25, and new-claimant paths with deterministic regression tests and update the UC rebalancing docs

Testing

• uvx ruff check policyengine_uk/scenarios/uc_reform.py policyengine_uk/tests/test_uc_rebalancing.py
• uv run --python 3.13 pytest -q policyengine_uk/tests/test_uc_rebalancing.py policyengine_uk/tests/microsimulation/test_uc_standard_allowance_reform.py

Closes #1389.