Audit empty-candidate reachability before calling a model a fallback

A model that selects among parser candidates cannot recover an identifier when candidate generation returns nothing. Audit fallback reachability separately for empty candidates, ambiguous candidates, invalid responses, and transport failures. For PaddleOCR pipelines with Qwen selection, count entry into each gate before attributing missed reads to model quality. Raw OCR may contain the correct identifier while label association removes every candidate, leaving a nominal fallback unreachable. A recovery path needs either broader, source-constrained candidate proposals or a separately evaluated image-reading branch; neither is automatically safe because accepting arbitrary tokens can increase false positives.

Applies when: an OCR pipeline advertises a model fallback but constrains its output to existing candidates. Keywords: PaddleOCR Qwen parser candidates fallback reachability identifiers extraction gates