Compile ACA bronze marketplace targets

[daphnehanse11]

Summary

• Add CMS ACA bronze APTC consumer facts to the US fiscal target compiler.
• Materialize less-than count targets, so selected_marketplace_plan_benchmark_ratio < 1.0 can be counted within the assigned_aca_ptc > 0 domain.
• Add regression tests for bronze target metadata and threshold-count domain filtering.

Validation

• uv run ruff check packages/populace-build/src/populace/build/us/fiscal_targets.py tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py packages/populace-build/tests/test_us_fiscal_targets.py
• uv run pytest packages/populace-build/tests/test_us_fiscal_targets.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py packages/populace-build/tests/test_us_source_runtime.py packages/populace-build/tests/test_us_source_coverage.py -q
• Compiling the cached current 2024 facts still yields only 51 ACA enrollment and 51 APTC-recipient targets, confirming the remaining gap is that the active facts bundle does not yet emit bronze/state-metal rows.

Refs #75

[daphnehanse11]

Updated this PR for the review finding:

• rebased onto current main
• classified measure_mode="less_than_count" as count-like in fiscal target loss-basis weighting
• classified aca_bronze_aptc_consumers consistently with ACA person-count targets
• added a regression for bronze ACA count basis

Verification:

• uv run pytest packages/populace-build/tests/test_us_fiscal_refresh_builder.py packages/populace-build/tests/test_us_fiscal_targets.py
• GitHub test/wheel checks are green for Python 3.13 and 3.14