Add CPS SPM takeup source inputs

[MaxGhenis]

Summary

Fills a distinct CPS/SPM slice of the remaining eCPS column gap without overlapping PR #125's G7 income-copy lane or PR #127's G6/G8/G9 recode lane.

• derive observed housing assistance and housing takeup from CPS SPM_CAPHOUSESUB
• derive Medicare takeup from CPS Medicare coverage
• carry CPS SPM_ENGVAL to spm_unit_energy_subsidy on the SPM-unit table and allow it as a data-overridable computed export
• add source, entity-table, and export-map regressions

Validation

• uv run --extra dev python -m pytest tests/test_cps_source_provider.py::test_load_cps_asec_derives_policyengine_value_inputs tests/pipelines/test_us.py::TestUSMicroplexPipeline::test_build_policyengine_entity_tables_preserves_spm_source_inputs tests/policyengine/test_us.py::TestPolicyEngineUSProjection::test_build_policyengine_us_export_variable_maps_includes_contract_inputs -q
• uv run --extra dev ruff check src/microplex_us/data_sources/cps.py src/microplex_us/microdata_roles.py src/microplex_us/pipelines/us.py src/microplex_us/policyengine/us.py tests/test_cps_source_provider.py tests/pipelines/test_us.py tests/policyengine/test_us.py

Note: I did not run file-wide ruff format --check because current origin/main would reformat unrelated existing lines in touched files; the patch intentionally avoids that churn.

Release gate

This should merge before the next CPS-spine rebuild, but it is not by itself enough to replace eCPS. Any policyengine-bundles release / policyengine.py dataset switch should wait for a fresh build plus sound eCPS comparison, reform benchmark, and reproducibility checks.