Describe held-out shapes, message content, and background images

README.md

@@ -76,6 +76,13 @@ field identifies the 75 IID and 75 OOD episodes in each Drag environment.
 Rotation and Ten-Choice each use fixed test sets without an IID/OOD subdivision,
 so their `distribution` values are `null`.
 
+The held-out factors follow the paper's test-set construction. Drag's IID
+episodes use training shape categories, while its OOD episodes use held-out
+shape categories. All Ten-Choice test scenes draw from 240 messages disjoint
+from the 80 training messages. Rotation uses 150 test background images
+disjoint from the 1,000 training backgrounds. Ten-Choice and Rotation therefore
+evaluate held-out content across their full test sets.
+
 ## Load episode records
 
 ```python