* fix(mount): keep periodic metadata flush from dropping concurrent chunk uploads
The periodic flush snapshotted entry.Chunks, then ran CompactFileChunks and
MaybeManifestize (the manifest upload is a network round trip) before
reassigning entry.Chunks. Async uploaders append freshly uploaded chunks
during that window, and the reassignment overwrote them: the data stayed on
the volumes but the file lost those chunk references, leaving zero-filled
holes on read. Large sequential writes such as cat of two 15 GiB files hit
several flush cycles and ended up corrupted.
Snapshot the chunk list under the entry lock with a length marker, do the
slow compaction and manifestization on the snapshot, then splice the
processed prefix back in front of whatever chunks arrived after the
snapshot.
* mount: drop redundant slice copies in the flush splice
processedPrefix is freshly built and the tail sub-slice is consumed
immediately under the entry lock, so append straight onto processedPrefix
instead of allocating two throwaway copies.
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