ci: bootstrap pin moves to 2026.9.3.1

The release is published, mirrored and indexed: openxlings/xim-pkgindex's
`latest` points at 2026.9.3.1 and that commit's `Publish Index Artifact` run is
green, which is what `xlings install` consumes. Verified in a clean sandbox
before this: `xlings install mcpp@2026.9.3.1` then `mcpp --version`, followed by
the eight-section verification of the release's own behaviour.

The pin is bumped only after that sequence — CI installs the pinned version
from the index, so a pin ahead of the index is a job that cannot start.

Author: speak-agent

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "2026.9.2.1"
+    "mcpp": "2026.9.3.1"
   }
 }