extract_page_blocks() walked `record.boundary` and `record.baseline`
unconditionally, so a record that arrived without either (malformed
kraken output, or a MagicMock in tests that iterates to nothing)
crashed with "min() arg is an empty sequence".
Coerce both attributes through list(), require at least 3 points for
the polygon path, fall back to the baseline path when the polygon is
missing, and skip the record entirely when neither is usable —
emitting no block is safer than emitting one with garbage coordinates.
The test helper now sets `boundary` and `baseline` explicitly to
mirror real Kraken 7.0 records (and so the happy-path test exercises
the polygon branch). A new regression test covers the skip path.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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