* s3: track manifest blob ownership through multipart completion
Manifest blobs made three orphan paths. A partial fold that failed midway
kept its earlier batches on volume servers while the write fell back to
flat chunks; the fold now records each saved blob and deletes them on
error. A completion that failed after preparing left its fresh manifests
behind on every retry; the completion state now owns them and deletes
them unless a failed rollback left the version entry still holding them.
And a completed upload removes its parts metadata-only, which stranded
the part-manifest blobs superseded by flattening; those are collected
during flattening and deleted once the completion commits.
Two shared-chunk hazards nearby: the version-file rollback deleted its
data, destroying the still-registered parts (worse once manifests
resolve to inner chunks), and the idempotent-replay cleanup data-deleted
leftover parts whose chunks the live object references. Both are
metadata-only now.
* s3: trim chunk manifest comments
* s3: test manifest fold rollback and part range selection
The fold-with-rollback and the boundary-to-byte-range logic were only
exercised by hand against a live server; give both an injectable seam
and cover the fold, the below-threshold and SSE no-ops, the midway
failure deleting the blobs it saved, and offset-vs-legacy-index range
selection including indexes that no longer address the chunk list.
