Add eCPS-style ACA take-up inputs

[MaxGhenis]

Summary

• preserve explicit ACA take-up inputs when present
• otherwise attach eCPS-style deterministic ACA take-up at the tax-unit level using the PolicyEngine US data take-up rate/RNG
• stop using observed marketplace coverage as a fallback proxy for ACA take-up

Validation

• .venv/bin/ruff check src/microplex_us/pipelines/us.py tests/pipelines/test_us.py
• .venv/bin/ruff format --check src/microplex_us/pipelines/us.py tests/pipelines/test_us.py
• .venv/bin/python -m pytest tests/pipelines/test_us.py -q