Watermark-based batch export script (scripts/export_corpus.py) pushes up to 500
ERROR/CRITICAL entries and labeled incidents per run to AVOCET_CORPUS_ENDPOINT.
Uses SQLite rowid watermark (entry log) and ISO timestamp watermark (incidents).
Skips silently when AVOCET_CORPUS_ENDPOINT is not set. 19 tests. Closes turnstone#6.
