ci: workspace mcpp bootstrap pin -> 2026.9.5.2 (released + indexed)

The pin must name a version that is published, mirrored and resolvable through
the index, which 2026.9.5.2 now is: both mirrors return 200 with matching byte
counts for all four artifacts, and `Publish Index Artifact` is green on the
xim commit that made it `latest`.

Author: speak-agent

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "2026.9.3.2"
+    "mcpp": "2026.9.5.2"
   }
 }