The store's garbage-collect marks only from manifests, and archives were
published as bare blobs, so the first real collection would delete every
archive the mesh keeps. PublishArchive now puts a deterministic OCI holder
manifest (empty config, one layer) beside each archive; the sweep holds every
kept archive before it lets anything go, which backfills existing bare blobs,
and lets go of an archive holder-first. A forgotten module no longer keeps its
five recent builds (ADR 0189). `collection [--json]` reports kept archives
held/unheld and what may be let go, so the dry run can be lifted on evidence.
