* fix(shell): fs.mergeVolumes now rewrites manifest chunks for large files
Previously fs.mergeVolumes skipped any chunk whose IsChunkManifest flag was
true, printing "Change volume id for large file is not implemented yet" and
continuing. Because the BFS traversal only looks at top-level
entry.Chunks, sub-chunks referenced inside a manifest were never
considered either. For any file stored as a chunk manifest (large files
go this path), chunks in the source volume stayed put, leaving behind a
few MB of live data that vacuum and volume.deleteEmpty couldn't clean
up.
This change resolves each manifest chunk recursively, moves any
sub-chunk whose volume id is in the merge plan via the existing
moveChunk path, and re-serializes the manifest. If the manifest chunk
itself lives in a source volume, or any sub-chunk moved, the new
manifest blob is uploaded to a freshly assigned file id (the old
needle becomes orphaned and is reclaimed by vacuum like any other
moved chunk).
Fixes#9116.
* address review: batch UpdateEntry, fix dry-run, defer restore, avoid source volumes
- Call UpdateEntry once per entry after the chunk loop instead of once per
moved chunk (gemini nit).
- In dry-run mode, mark anySubChanged when a sub-chunk in the plan is
encountered and return changed=true after printing "rewrite manifest",
so nested manifests also surface their would-rewrites (gemini nit).
- Defer filer_pb.AfterEntryDeserialization so the manifest chunk list is
restored even when proto.Marshal fails (coderabbit nit).
- Reject AssignVolume results whose file id lands on a volume that is a
source in the merge plan, and retry — otherwise the replacement
manifest could be written to the volume being emptied (coderabbit).
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