Emit ACA metal-level release diagnostics

[daphnehanse11]

Summary

• Add ACA metal-level diagnostics to the release calibration diagnostics build payload.
• Pair below-benchmark/bronze metal targets with APTC recipient denominators by state and nationally, including shares, missing diagnostics, missing denominators, and largest state share errors.
• Add focused builder tests for state/national share aggregation and the unavailable no-metal-target path.

Closes #75

Tests

• uv run pytest packages/populace-build/tests/test_us_fiscal_refresh_builder.py -q
• uv run ruff check tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py
• uv run ruff format --check tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py
• git diff --check