Activate ACA state metal targets

[daphnehanse11]

Summary

• Keeps the ACA source refresh on the full selected-plan calibration path when bronze/metal targets are present, instead of stopping at support clipping.
• Adds selected-marketplace-plan benchmark-ratio diagnostics to the health-input signal gate, including support bounds and all-tax-unit / Marketplace-taker below-benchmark counts.
• Adds regression coverage that bronze targets flow through the existing ACA state-level source path without activating an unregistered cms-aca-oep-state-metal source-package alias.

Notes

This progresses #75. The Populace release path now consumes cms_aca.oep2024.state_metal.*.bronze_aptc_consumers targets when they are present in the Ledger facts, but it does not declare a separate cms-aca-oep-state-metal package alias because I could not verify that alias in the current Ledger source-package registry.

Validation

• uv run pytest packages/populace-build/tests/test_us_source_runtime.py packages/populace-build/tests/test_us_fiscal_targets.py packages/populace-build/tests/test_us_plan.py packages/populace-build/tests/test_us_source_coverage.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py
• uv run ruff check packages/populace-build/src/populace/build/us/source_coverage.py packages/populace-build/src/populace/build/us/__init__.py tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_source_coverage.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py
• uv run ruff format --check packages/populace-build/src/populace/build/us/source_coverage.py packages/populace-build/src/populace/build/us/__init__.py tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_source_coverage.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py
• git diff --check