Expose axes in household calculators

[MaxGhenis]

What changed

• Adds an axes= keyword to pe.us.calculate_household and pe.uk.calculate_household.
• Accepts either the lower-level nested axes shape or a flat list of axis dictionaries.
• Defaults missing axis period values to the household calculator year.
• Returns axis-series lists instead of scalars when axes are present, while preserving per-person result objects.
• Documents the new household axes API and adds US/UK regression coverage.

Why

GiveCalc needs to evaluate the same household across donation amounts. Exposing axes through the public policyengine.py household calculators avoids forcing applications to reach into country-package Simulation classes directly.

Checks

• uv run ruff check src/policyengine/tax_benefit_models/common src/policyengine/tax_benefit_models/us/household.py src/policyengine/tax_benefit_models/uk/household.py tests/test_household_impact.py
• uv run ruff format --check src/policyengine/tax_benefit_models/common src/policyengine/tax_benefit_models/us/household.py src/policyengine/tax_benefit_models/uk/household.py tests/test_household_impact.py
• uv run --extra dev pytest tests/test_household_impact.py -q
• uv run --extra dev pytest tests/test_household_calculator_snapshot.py -q