fix: Initialize imgs list in crop() fallback path

- Critical bug fix: imgs list was not initialized before use (line 439)
- Without this fix, NameError would occur when cache miss triggers fallback
- Discovered during reliability audit of MinerU image generation fix

deepdoc/parser/mineru_parser.py

@@ -436,6 +436,7 @@ class MinerUParser(RAGFlowPdfParser):
         )
 
         positions = []
+        imgs = []
         for ii, (pns, left, right, top, bottom) in enumerate(poss):
             right = left + max_width