ci: workspace mcpp bootstrap pin -> 2026.8.3.2 (released + indexed)

Safe to move now: v2026.8.3.2 is published on all four platforms, mirrored to
xlings-res, and xim-pkgindex tracks it as latest (openxlings/xim-pkgindex#480).
Verified end to end — `xlings install mcpp@2026.8.3.2` then `xlings use`
resolves to 2026.8.3.2.

The pin is the self-hosting starting point, so it only ever moves AFTER the
release it names actually exists and is reachable through the index; bumping it
alongside the version bump would have pointed every CI job at a release that had
not been built yet.

Author: Sunrisepeak

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "2026.8.3.1"
+    "mcpp": "2026.8.3.2"
   }
 }