feat: Add layout complexity detection (tables and multi-column)

Add LayoutComplexity struct to PdfProcessResult so callers can detect
when a PDF has complex layout (tables or multi-column text) and decide
whether to use the extracted markdown or fall back to OCR.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/integration_tests.rs

@@ -948,6 +948,7 @@ fn test_pages_needing_ocr_field_accessible() {
         pages_needing_ocr: vec![1, 3],
         title: None,
         confidence: 1.0,
+        layout: pdf_inspector::LayoutComplexity::default(),
     };
     assert_eq!(process_result.pages_needing_ocr, vec![1, 3]);
 }