Keep CD targets diagnostic for small releases

[MaxGhenis]

Summary

• keep congressional-district fiscal targets diagnostic-only by default for the small dataset release path
• add --gate-congressional-district-targets for future support-rich CD builds that should hard-gate on CD rows
• exclude diagnostic-only CD rows from dropped/skipped/zero-support release gates without removing them from the target surface or diagnostics
• avoid storing the full CD target-name list in diagnostics; only dropped diagnostic-only names are recorded

Validation

• uv run --python 3.13 ruff format tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py
• uv run --python 3.13 ruff check tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py
• uv run --python 3.13 --package populace-build --with pytest python -m pytest packages/populace-build/tests/test_us_fiscal_refresh_builder.py -q
• git diff --check