Expose axes in household calculators

[MaxGhenis]

Summary

• add axes support to the US and UK calculate_household helpers
• normalize flat or nested axis specs and default missing periods to the household calculation year
• return per-axis series for person and group entity results

Why

The app and Python household calculators need the same axis-grid behavior for simple one-household charts. The previous draft PR for this work, #322, is conflict-stuck against current main; this branch refreshes the same useful change onto the current package surface.

Validation

• bash .github/check-changelog.sh
• ruff format --check .
• ruff check .
• uv run --extra dev python -m pytest tests/test_household_impact.py -q

[MaxGhenis]

Disposition after full merits review: rebase-and-revive — done. The branch was MERGEABLE and its own 90 lines of axes tests all passed in CI; the only red was tests/test_models.py's label assertion, which main fixed with a tolerant regex (ab38116) about four hours after this branch was cut. Rebased onto current main (no conflicts — the patch-target files were untouched since May) and hardened the one future-brittle assertion (exact income-tax liabilities → the donations-reduce-tax relationship). 68/68 tests pass locally against policyengine-us 1.752.2, including empirical confirmation that every model-version attribute normalize_axes reads exists on the current pin. Design reviewed against the repository public-surface rules: rides the wrapper Simulation (no core bypass), JSON-serialisable list values, kwarg allowlist + error message updated in lockstep. CI re-running now.