Preserve ETERNITY inputs across cache invalidation

[anth-volk]

Fixes #482

Summary

• Preserve user inputs through storage get/put so ETERNITY period canonicalization is honored during cache invalidation.
• Add regressions for Simulation.set_input and direct Holder.set_input.

Tests

• uv run pytest tests/core/test_apply_reform_preserves_user_inputs.py
• uv run pytest tests/core/test_apply_reform_invalidates_cache.py tests/core/test_subsample_invalidates_cache.py
• uv run pytest tests/core/test_apply_reform_preserves_user_inputs.py tests/core/test_apply_reform_invalidates_cache.py tests/core/test_subsample_invalidates_cache.py tests/core/test_eternity_subperiods.py tests/core/test_holders.py tests/core/test_storage_delete_branch.py
• uv run ruff check policyengine_core/simulations/simulation.py tests/core/test_apply_reform_preserves_user_inputs.py
• uv run ruff format --check policyengine_core/simulations/simulation.py tests/core/test_apply_reform_preserves_user_inputs.py