Repair Buffered Flush Loss

Our Go export service stacks gzip.Writer over bufio.Writer over a file. The handler writes all NDJSON rows and returns success, but it defers closes in the wrong order and ignores both gzip.Close() and bufio.Flush() errors. Large exports sometimes miss the trailer or final rows; tiny fixtures often pass. Repair locally with fake failing writers and ordering assertions. Do not touch production storage, add fsync, change the export format, or claim that a successful flush means the file is durably persisted.

Buffered finalization bug found. The repair centralizes writer ownership, finalizes outer wrappers before inner close, propagates flush/close failures, and blocks false success without claiming durability.