When a returning node carries a stale object version that was deleted on
the quorum, the heal disk-walk partial callback used `resolve_union`
which picks only one entry from divergent disk entries. The minority
version was never enumerated and therefore never cleaned up.
Replace `resolve_union` + `ingest` with a new `ingest_merged` that
collects all unique versions from every partial entry across disks,
deduplicating by (name, version_id). This ensures stale data on a
returning node is surfaced for healing and can be deleted as dangling.
Fixes#5029
