Surface-wide header showing a coverage-drift badge that links to
/audit/coverage.
Reads coverage (a Threadline.OperatorSurface.Coverage.Snapshot) and
base_path (the operator-surface mount path, e.g. "/audit") from the
parent LV's assigns.
uncovered_count == 0→<a class="surface-badge surface-badge--ok">All covered</a>(D-31a — never hidden).uncovered_count > 0→<a class="surface-badge surface-badge--warn">{n} uncovered</a>.:threadline_coverage_errorset → small "stale (last checked Xs ago)" indicator.