Round tapered personal allowance up to nearest pound

[MaxGhenis]

Summary

• round tapered Personal Allowance amounts up to the nearest pound
• add a zero-margin 2026 regression case for a fractional taper amount
• add the changelog entry for Round tapered personal allowance up to nearest pound #1738

Notes

The formula stays vectorized: it applies numpy.ceil to the array returned by max_(0, personal_allowance - reduction) rather than using scalar control flow.

Fixes #1738.

Tests

• uv run ruff check policyengine_uk/variables/gov/hmrc/income_tax/allowances/personal_allowance.py
• uv run policyengine-core test policyengine_uk/tests/policy/baseline/gov/hmrc/income_tax/allowances/personal_allowance.yaml -c policyengine_uk
• git diff --check

[MaxGhenis]

Closing as superseded by #1751, which merged the tapered Personal Allowance rounding fix and closed #1738. This older branch now conflicts with main.