reporter_flush() snapshotted the buffers under lock, released the lock
to POST, then unconditionally cleared the entire buffer on success.
Records appended by reporter_submit_*() during the in-flight POST were
silently erased. Replace clear() with erase() of just the snapshotted
prefix so concurrent appends survive.
Found via adversarial review (gpt-5.5 reviewer, run 2026-05-01-190903).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Peter Woolery
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