Update ACA take-up time series

[daphnehanse11]

Summary

• Add ACA take-up values for 2023 through 2026 using APTC enrollment, not total Marketplace plan selections.
• Compute each rate as APTC enrollment divided by APTC enrollment plus the 2023 tax-credit-eligible uninsured count.
• Add a regression test for year-specific load_take_up_rate("aca", year) values.

Sources

• KFF State Health Facts: Distribution of Eligibility for ACA Health Coverage Among the Remaining Uninsured
• CMS: Effectuated Enrollment: Early 2024 Snapshot and Full Year 2023 Average
• CMS: 2024 Marketplace Open Enrollment Period Public Use Files
• CMS: Effectuated Enrollment: Early 2025 Snapshot and Full Year 2024 Average
• CMS: 2026 Marketplace Open Enrollment Period Public Use Files

Open reviewer question

This keeps 2021 and 2022 on the existing baseline because the denominator source is 2023-specific. If reviewers are comfortable backcasting the 2023 denominator, we can extend the same calculation to 2021 and 2022 in a follow-up or in this PR.

Tests

• uv run pytest tests/unit/test_stochastic_variables.py -q
• uv run ruff check tests/unit/test_stochastic_variables.py
• git diff --check
• uv run towncrier check --compare-with origin/main

Addresses #620.