Derive ACA takeup from marketplace coverage

[MaxGhenis]

Summary

• derive takes_up_aca_if_eligible at the tax-unit level from observed marketplace coverage columns instead of letting the writer default every tax unit to true
• preserve explicit takes_up_aca_if_eligible inputs when present
• add regression coverage for inferred and explicit ACA takeup

Evidence

• Probe artifact: /Users/maxghenis/CosilicoAI/microplex-us/artifacts/aca_takeup_from_marketplace_probe_20260530/aca_takeup_probe.json
• Existing small MP H5 had takes_up_aca_if_eligible=True for 130,980 / 130,980 tax units
• Applying the new rule to the same rows marks 5,886 / 130,980 tax units true, 4.35% weighted, from observed marketplace coverage

Validation

• uv run --extra dev ruff check src/microplex_us/pipelines/us.py tests/pipelines/test_us.py
• git diff --check
• uv run --extra dev --extra policyengine pytest tests/pipelines/test_us.py -k "aca_takeup or augment_policyengine_person_inputs"
• uv run --extra dev --extra policyengine pytest tests/pipelines/test_us.py
• uv run --extra dev --extra policyengine pytest tests/policyengine/test_us.py::TestPolicyEngineUSProjection::test_default_policyengine_us_export_surface_avoids_formula_aggregates tests/policyengine/test_us.py::TestPolicyEngineUSProjection::test_writer_rejects_computed_policyengine_variables tests/pipelines/test_mp300k_artifact_gates.py -k "ecps_comparison or column_contract"

Refs #86