Detect broken font encodings and flag for OCR fallback

Add has_encoding_issues field to PdfProcessResult that detects garbled
text from broken ToUnicode CMaps (U+FFFD replacement characters or
systematic dollar-as-space substitution). Surfaced in JSON output so
clients can fall back to OCR for affected PDFs.

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

tests/integration_tests.rs

@@ -950,6 +950,7 @@ fn test_pages_needing_ocr_field_accessible() {
         title: None,
         confidence: 1.0,
         layout: pdf_inspector::LayoutComplexity::default(),
+        has_encoding_issues: false,
     };
     assert_eq!(process_result.pages_needing_ocr, vec![1, 3]);
 }