Fix UC standard allowance reforms being ignored

[MaxGhenis]

Summary

• move the rebalancing uplift into the uc_standard_allowance formula
• stop overriding uc_standard_allowance via scenario set_input so user reforms still apply
• add a focused microsimulation regression test

Context

• supersedes the stale conflicting branch in Fix UC standard allowance reforms being ignored (#1472) #1507 with a fresh re-cut on current main
• intentionally leaves out the stale reforms_config.yaml edits from the old PR

Validation

• uv run pytest policyengine_uk/tests/microsimulation/test_uc_standard_allowance_reform.py -q