Add query-string subset diagnostic

README.md

@@ -262,6 +262,28 @@ The published-subset advantage is larger at 10.6 points
 (-5.2 to 3.6), providing no clear accuracy advantage over TF-IDF.
 Only one training seed was evaluated.
 
+### Query-string shortcut check
+
+No legitimate training examples contained query strings. To check
+whether gains were limited to flagging query-containing URLs, a
+post-hoc analysis compared models on test URLs without queries.
+
+- **Internal:** all 14 net additional correct predictions over TF-IDF
+  came from URLs without queries. On this subset, QLoRA's accuracy
+  advantage was 3.59 percentage points (95% paired-bootstrap interval:
+  0.77 to 6.41).
+- **Published subset:** 50 of 53 net additional correct predictions
+  over TF-IDF came from URLs without queries. The no-query advantage
+  was 10.53 points (7.16 to 13.89).
+- **External:** no-query accuracy remained poor: 54.27% for QLoRA
+  versus 55.77% for TF-IDF.
+
+Simply flagging query-containing test URLs cannot explain the
+familiar-source gains. This does not establish causal independence
+from query-related learning or rule out other shortcuts. Intervals
+use 5,000 paired, source/class-stratified bootstrap replicates and
+are not adjusted for multiple comparisons or training-seed variability.
+
 ## Release verification
 
 The uploaded adapter and inference script were verified at revision