add region-based text extraction for hybrid OCR pipelines

Add `extract_text_in_regions_mem` that takes layout-detected bounding
boxes (top-left origin, PDF points) and returns text within each region
in reading order. Designed for pipelines where a layout model detects
regions and text-based pages can skip GPU OCR by extracting text from
the PDF structure directly.

Also add `classify_pdf_mem` for lightweight PDF type classification
returning 0-indexed pages_needing_ocr.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/extractor/mod.rs

@@ -2,7 +2,7 @@
 //!
 //! This module extracts text with position information for structure detection.
 
-mod content_stream;
+pub(crate) mod content_stream;
 mod fonts;
 mod layout;
 mod links;