Decode enum values in US household results

[anth-volk]

Fixes #459

Summary

• Request decoded enum values from the US country model when assembling household results.
• Preserve the existing numerical result conversion.
• Add regression coverage for scalar and axes-based enum outputs.
• Update household snapshots to record enum names instead of numeric indices.

Testing

• make format
• make lint
• uv run pytest tests -q --disable-warnings --cov=policyengine --cov-report= (1,116 passed, 9 skipped; four expected enum snapshot mismatches subsequently updated)
• uv run pytest tests/test_household_calculator_snapshot.py tests/test_household_impact.py -q --disable-warnings (47 passed)
• uv run mypy src/policyengine is blocked by an installed NumPy type-definition syntax error before project code is checked.