A long-running install can otherwise grow the upload log into the
gigabyte range, eating disk and slowing every appendFile. Add a
size-checked rotation right before each flush:
- statSync the resolved log target (cheap, ENOENT skips silently).
- If size exceeds 50 MB, drop the oldest backup (.3), shift .2→.3
and .1→.2, then rename the live file to .1 and let appendFile
create a fresh primary on the next call.
- Max 3 backups (~200 MB worst case, bounded). debugLog records
each rotation for diagnostics.
- Pure additive: skips when file is small or doesn't exist; no
effect on the daily-log mode (already date-rotated).
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