ci-impact: fail-safe except branch untested (coverage-004)

[SUaDtL]

Branch: feat/pi-support · Tribunal run 2026-07-17-pi-support-branch · Finding: findings/coverage/coverage-004.json
tools/ci-impact.py:422-448 fall-back-to-broad-validation branch never exercised; ReceiptCommandTest covers only success.

[SUaDtL]

Verified against PR #313 head 9ed248f4: .github/scripts/test_ci_impact.py now drives main() through the fail-safe exception path and asserts a broad-validation receipt with fallback: true; success-path receipt coverage remains intact. The hosted impact-selection and impact-planner checks are green. This finding is implemented on #313, which now carries Closes #323.