ci: workspace mcpp bootstrap pin -> 2026.8.3.1 (released + indexed) (#338)

Co-authored-by: sunrisepeak <speakshen@163.com>

Author: speak-agent

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "2026.8.2.2"
+    "mcpp": "2026.8.3.1"
   }
 }