ci: bootstrap with mcpp 2026.11.3.1 (#886)

Advance the workspace bootstrap pin to mcpp 2026.11.3.1 after its immutable release, mirrors, published index and isolated installation checks completed.

All final-head checks passed or were intentionally skipped. Infrastructure-only interruptions were rerun; the complete compiler, unit, E2E and cross-platform gates remain intact.

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

Author: Sunrisepeak

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "2026.10.19.1"
+    "mcpp": "2026.11.3.1"
   }
 }