Round tapered personal allowance up

[MaxGhenis]

Summary

• Round tapered personal allowance amounts up to the nearest pound after the high-income taper
• Add a 2026 regression test for a fractional taper remainder

Fixes #1738.

Tests

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

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