Adds CoverageRollup type, getCoverageRollup() against
/api/admin/library/coverage, and createCoverageQuery() with the same
3s/30s pacing used by scan-status. New qk.coverage() key follows the
existing zero-arg tuple pattern. Vitest covers the GET path and the
empty-library zero-state plus the with_art + pending + settled = total
invariant.