Add FRS child and qualifying young person inputs

[MaxGhenis]

Summary

• derive receives_benefits_in_own_right from reported Universal Credit, JSA, and ESA receipt in the FRS person table
• add FRS person-level inputs for current non-advanced education, approved training, education/training entry age, and the Universal Credit age-19 terminal-date proxy
• pair this data wiring with Tighten child and qualifying young person benefit rules policyengine-uk#1655 so the legal child/QYP predicates can use microdata inputs instead of country-package defaults

Notes

• FRS does not observe exact course acceptance/start age or UC assessment-period terminal-date detail, so the PR makes those explicit data-side proxies for observed current qualifying education/training.
• Government-training codes exclude the FRS None of these codes.

Tests

• uv run pytest policyengine_uk_data/tests/test_legacy_benefit_proxies.py -q
• uv run ruff check policyengine_uk_data/datasets/frs.py policyengine_uk_data/tests/test_legacy_benefit_proxies.py
• uv run ruff format --check policyengine_uk_data/datasets/frs.py policyengine_uk_data/tests/test_legacy_benefit_proxies.py
• git diff --check