feat: Add adds/subtracts to Variable with parameter path resolution

[anth-volk]

Fixes #247

Summary

• Adds adds and subtracts fields (list[str] | None) to the Variable Pydantic model
• Extracts these from policyengine-core variable objects during US and UK model initialization
• Resolves parameter path strings (e.g., "gov.hhs.tanf.cash.income.sources.earned") to list[str] so downstream consumers always get arrays
• Adds 9 unit tests covering list adds, parameter-path resolution, subtracts, None cases, and cross-referencing validity

Test plan

• TestVariableAddsSubtracts::test_us_variable_with_list_adds_has_list — direct list adds (e.g., employment_income)
• TestVariableAddsSubtracts::test_us_variable_with_parameter_path_adds_resolves_to_list — parameter path resolution (e.g., household_state_benefits)
• TestVariableAddsSubtracts::test_us_variable_without_adds_has_none — variables without adds
• TestVariableAddsSubtracts::test_us_variable_without_subtracts_has_none — variables without subtracts
• TestVariableAddsSubtracts::test_us_some_variables_have_adds — at least 50 US variables have adds
• TestVariableAddsSubtracts::test_uk_variable_with_adds_has_list — UK variable with adds
• TestVariableAddsSubtracts::test_uk_variable_without_adds_has_none — UK variable without adds
• TestVariableAddsSubtracts::test_us_variable_with_subtracts_has_list — US variable with subtracts
• TestVariableAddsSubtracts::test_adds_entries_are_valid_variable_names — adds entries reference real variables

🤖 Generated with Claude Code

[nwoodruff-co]

Why is this needed- and if so, should probably be proper references to other rows in the Variable table

[anth-volk]

This is needed to build the results tree we use for nested household outputs. You make a fair point about row references, though I'll need to think through implementation. I wonder what the time and complexity costs of creating them would be, considering that you'd need to create rows that are within a given var's adds or subtracts first, then create the "consumers", so to speak.

[anth-volk]

I did some looking into this and I can't envision an implementation where we don't have to algorithmically determine which params to load in first based on where they sit in the adds/subtracts tree, especially since they can nest pretty deeply in -us.