ci: workspace mcpp bootstrap pin -> 2026.9.12.4 (released + indexed) (#628)

Co-authored-by: speak-agent <248744407+speak-agent@users.noreply.github.com>

Author: Sunrisepeak

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "2026.9.12.3"
+    "mcpp": "2026.9.12.4"
   }
 }