Populate prompt_template_source for all subsets per 2026-05-30 audit

metadata.json

@@ -48,6 +48,10 @@
             "test": "https://huggingface.co/datasets/jeepliu/VisualMRC"
           }
         }
+      },
+      "prompt_template_source": {
+        "origin": "fallback",
+        "reference": "T2"
       }
     }
   }