Assign is_parent from FRS microdata

[MaxGhenis]

Summary

• Assign is_parent directly from FRS adult-table membership plus benefit-unit dependent-child counts.
• Avoid ranking adults across a whole household when multiple benefit units share one household.
• Add focused helper tests and extend the FRS smoke test to assert the column is produced.
• Sync the editable package version in uv.lock with pyproject.toml.

Closes #73.

Impact check

Using local raw FRS microdata and approximating the old formula:

Dataset	Persons	New FRS parents	Old formula parents	Differing persons	New true / old false	Old true / new false	Diffs in multi-BU households
frs_2022_23	53,577	10,901	10,396	561	533	28	299
frs_2023_24	36,248	7,429	7,119	352	331	21	206

Validation

• uv run --python 3.13 ruff check policyengine_uk_data/datasets/frs.py policyengine_uk_data/tests/test_is_parent_from_frs.py policyengine_uk_data/tests/test_legacy_benefit_proxies.py
• uv run --python 3.13 ruff format --check policyengine_uk_data/datasets/frs.py policyengine_uk_data/tests/test_is_parent_from_frs.py policyengine_uk_data/tests/test_legacy_benefit_proxies.py
• uv run --python 3.13 pytest -q policyengine_uk_data/tests/test_is_parent_from_frs.py policyengine_uk_data/tests/test_legacy_benefit_proxies.py