Add direct CHIP enrollment targets

[daphnehanse11]

Summary

• map direct CMS CHIP enrollment facts to chip_enrolled when Ledger exposes chip_enrollment or total_chip_enrollment
• derive a first-class CHIP enrollment target from CMS Medicaid+CHIP minus Medicaid controls when only the current split facts are present
• require direct CHIP enrollment in the US fiscal target coverage gate so the Medicaid/CHIP split is visible in release diagnostics

Closes #139

Tests

• uv run pytest packages/populace-build/tests/test_us_fiscal_targets.py -q
• uv run pytest packages/populace-build/tests/test_us_fiscal_refresh_builder.py -q
• uv run ruff check packages/populace-build/src/populace/build/us_runtime/fiscal_targets.py packages/populace-build/tests/test_us_fiscal_targets.py
• uv run ruff format --check packages/populace-build/src/populace/build/us_runtime/fiscal_targets.py packages/populace-build/tests/test_us_fiscal_targets.py
• git diff --check