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

metadata.json

@@ -42,6 +42,11 @@
             "from": "Category"
           }
         }
+      },
+      "prompt_template_source": {
+        "origin": "fallback",
+        "reference": "T3-adapted",
+        "notes": "MCQ-shape per-dataset adaptation"
       }
     },
     "caption2figure": {
@@ -84,6 +89,11 @@
             "from": "Category"
           }
         }
+      },
+      "prompt_template_source": {
+        "origin": "fallback",
+        "reference": "T3-adapted",
+        "notes": "MCQ-shape per-dataset adaptation"
       }
     }
   }