Fix rent and childcare export inputs

[MaxGhenis]

Summary

• materialize pre_subsidy_rent from ACS-imputed rent before PE formula materialization, so formula-owned rent does not overwrite the donor value to zero
• export rent through the storable PE/eCPS pre_subsidy_rent input when building H5 arrays, while still excluding formula-owned rent itself
• bump the CPS ASEC processed-cache version and require spm_unit_pre_subsidy_childcare_expenses in cached person rows so stale pre-Populate rent (ACS donor) and childcare (CPS ASEC) — close Gate-2 zero-targets #180 caches rebuild
• add regressions for the pre-materialization rent alias, export alias, and childcare cache surface

Validation

• uv run --python 3.13 --extra dev --extra policyengine pytest tests/policyengine/test_us.py::TestPolicyEngineUSProjection tests/test_cps_source_provider.py tests/pipelines/test_us.py::TestUSMicroplexPipeline::test_augment_policyengine_person_inputs_aliases_rent_to_pre_subsidy_rent tests/pipelines/test_us.py::TestUSMicroplexPipeline::test_build_policyengine_entity_tables_preserves_spm_source_inputs
• uv run --python 3.13 --extra dev ruff check src/microplex_us/data_sources/cps.py src/microplex_us/policyengine/us.py src/microplex_us/pipelines/us.py tests/test_cps_source_provider.py tests/policyengine/test_us.py tests/pipelines/test_us.py
• uv run --python 3.13 --extra dev ruff format --check src/microplex_us/data_sources/cps.py src/microplex_us/policyengine/us.py src/microplex_us/pipelines/us.py tests/test_cps_source_provider.py tests/policyengine/test_us.py tests/pipelines/test_us.py
• checkpoint subset proof from failed Age SIPP and SCF donors to target year #185 RC: 500 nonzero post-imputation rent rows become 500 nonzero pre_subsidy_rent rows after _augment_policyengine_person_inputs, max preserved at 63,600

Release-candidate context

The #185 RC build passed export-column parity but failed the manual support audit because pre_subsidy_rent and spm_unit_pre_subsidy_childcare_expenses were all zero in the H5. The source stage had nonzero ACS-imputed rent, but post-microsim rent was all zero because PE materialization recomputed formula-owned rent from missing pre_subsidy_rent. Childcare was parsed from CPS but the active build reused the pre-#180 processed CPS cache.