* feat(mount): add singleflight deduplication for concurrent chunk reads
When multiple FUSE readers request the same uncached chunk concurrently,
only one network fetch is performed. Other readers wait and share the
downloaded data, reducing redundant volume server traffic under parallel
read workloads.
* fix(util): make singleflight panic-safe with defer cleanup
If the provided function panics, the WaitGroup and map entry are now
cleaned up via defer, preventing other waiters from hanging forever.
* fix(filer): remove singleflight from reader_cache to fix buffer ownership
The singleflight wrapper around chunk fetches returned the same []byte
buffer to concurrent callers. Since each SingleChunkCacher owns and
frees its data buffer in destroy(), sharing the same slice would cause
a use-after-free or double-free with the mem allocator.
The downloaders map already deduplicates in-flight downloads for the
same fileId, so the singleflight was redundant at this layer. The
SingleFlightGroup utility is retained for use elsewhere.
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