In SetHistory and Compact, the JSONL file was rewritten before updating
the meta file. If the process crashed between the two writes, the meta
still had a large Skip value pointing past the now-shorter JSONL file,
causing GetHistory to return empty — effectively data loss.
Reverse the order: write meta (with Skip=0) first, then rewrite JSONL.
On crash between the two writes, the old uncompacted file is still
intact and GetHistory reads from line 1, returning stale-but-complete
data. The next operation self-corrects.
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