ci: workspace mcpp bootstrap pin -> 2026.9.11.4 (#616)

2026.9.11.4 is published on all four platforms, byte-identical on both
xlings-res mirrors, and resolvable from the published index artifact: the
pointer names xim-pkgindex b28462a7 and all three mcpp platform blocks in that
artifact read latest = 2026.9.11.4. The pin moves only after that reading,
because a pin ahead of the artifact fails every bootstrap in CI.

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

Author: Sunrisepeak

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "2026.9.9.1"
+    "mcpp": "2026.9.11.4"
   }
 }