Write cache entries to a temporary file and rename them over the final
path only after the entry has been fully flushed. This keeps previously
mapped cache bodies tied to their original inode instead of exposing
them to truncation when the same cache key is replaced.
Clean up only the temporary file for incomplete writers so a failed
replacement does not remove the still-valid existing cache entry. Add
coverage that replaces an entry while an old body mapping remains live
and verifies that it still exposes the original bytes.
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