Match Qwen reasoning history across effort changes

src/engine/metal/qwen_mtplx/issue111_product.rs

@@ -98,8 +98,11 @@ fn issue111_token_prefix_cache() {
     assert_eq!(cold.cached_tokens, 0);
     assert_eq!(cold.prompt_tokens as usize, ids.len());
     assert!(owner.tokens().starts_with(&ids));
+    // Preserve the completed-turn tag: a fabricated tag forces a raw-history
+    // rebuild and does not exercise compatible live-generation continuation.
+    let tag = owner.checkpoint_tag();
     owner
-        .save_checkpoint(&directory.join("new-prefix.bin"), [111; 32], &mut |_| {})
+        .save_checkpoint(&directory.join("new-prefix.bin"), tag, &mut |_| {})
         .unwrap();
     let mut next = messages.clone();
     next.push(cold.message.clone());