Add dataset input contract helpers

[MaxGhenis]

Summary

• add a PolicyEngine-US-owned dataset input contract helper module for data-generation packages
• classify stochastic status inputs, medical/geographic/identifier inputs, deprecated aliases, and formula overrides such as FLSA overtime premium
• expose helpers to distinguish dataset-exportable variables from formula-owned outputs
• correct the WIC category comment to say family, matching the existing formula

Notes

• This does not add any dependency on policyengine-us-data and does not change formulas or default values.
• Downstream builders such as Microplex can use this after bumping their pinned policyengine-us version, while still owning their own data-generation logic.

Tests

• uv run ruff check policyengine_us/data/dataset_input_contract.py policyengine_us/data/init.py policyengine_us/tests/core/test_dataset_input_contract.py policyengine_us/variables/gov/usda/wic/wic_category.py
• uv run ruff format --check policyengine_us/data/dataset_input_contract.py policyengine_us/data/init.py policyengine_us/tests/core/test_dataset_input_contract.py policyengine_us/variables/gov/usda/wic/wic_category.py
• uv run pytest policyengine_us/tests/core/test_dataset_input_contract.py policyengine_us/tests/core/test_input_variable_definitions.py -q
• uv run pytest policyengine_us/tests/core/test_dataset_input_contract.py -q

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fa285ae) to head (5d8f174).
⚠️ Report is 26 commits behind head on main.

Additional details and impacted files

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- Hits            62        24   -38     

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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