Address PR #863 round 3:
- classify_from_text emits the "scanned" flag (was "no_text_layer") for the
empty-text-layer case -- same name + meaning as classify_pdf, so
astrolabe_document_classifier_flag_total isn't split across two labels for the
same concept (and matches the metric's documented vocab).
- classify_from_text logs at DEBUG when image_coverage length != the expected
min(pages, MAX_SAMPLED_PAGES), so a 1:1-alignment contract break (extractor
reorders/skips pages) surfaces instead of silently misattributing coverage.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
