Tighten disability category thresholds

[MaxGhenis]

Summary

• replace the disability-category 10% threshold discount with a fixed GBP 1/week tolerance
• document the tolerance as rounding protection rather than an empirical category shift
• update tests to prove near-threshold values still classify while 9-10% below-threshold values no longer get promoted

Closes #401

Tests

• uv run --python 3.13 --frozen ruff format --check policyengine_uk_data/datasets/disability_benefits.py policyengine_uk_data/tests/test_disability_benefits.py
• uv run --python 3.13 --frozen ruff check policyengine_uk_data/datasets/disability_benefits.py policyengine_uk_data/tests/test_disability_benefits.py
• uv run --python 3.13 --frozen pytest policyengine_uk_data/tests/test_disability_benefits.py -q
• uv run --python 3.13 --frozen pytest policyengine_uk_data/tests/test_frs_only_imputation.py::test_frs_only_recomputes_disability_flags_after_amount_imputation -q
• git diff --check