Run ACA source refresh in fiscal release builder

[MaxGhenis]

Summary

• runs the manifest-backed ACA Marketplace source refresh inside the US fiscal release builder before the health-input gate and calibration
• derives source-runtime inputs from the release frame plus PolicyEngine-calculated eligibility, premiums, APTC, and SLCSP
• calibrates ACA take-up using eligible-person mass for person-count APTC targets, avoids treating observed Marketplace enrollment as a PTC fallback, and keeps missing subsidized anchors false
• adds regression coverage for multi-person ACA target grain and enrollment-only rejection

Verification

• env -u UV_FROZEN uv run --all-packages --all-groups ruff format --check tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py
• env -u UV_FROZEN uv run --all-packages --all-groups ruff check tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py
• env -u UV_FROZEN uv run --package populace-build --extra us python -m pytest packages/populace-build/tests/test_us_fiscal_refresh_builder.py packages/populace-build/tests/test_us_source_runtime.py -q
• env -u UV_FROZEN uv run --package populace-build --extra us python -m pytest packages/populace-build/tests -q
• local base-pool smoke: health input unique counts changed from {takes_up_aca_if_eligible: 1, selected_marketplace_plan_benchmark_ratio: 1} to {takes_up_aca_if_eligible: 2, selected_marketplace_plan_benchmark_ratio: 140}

Notes

This does not run a full fiscal refresh solve by itself; it removes the blocker where the release builder ignored the already-merged ACA source runtime, so the next full build can actually change the exported ACA inputs and downstream loss.