chore: bootstrap from 2026.9.9.1

The pin names the released mcpp that `xlings install mcpp` puts in the workspace
so CI can build mcpp from source. Its one hard constraint is direction: it must
never name a version that is not yet installable. 2026.9.9.1 is published,
mirrored on both hosts, merged into xim-pkgindex and confirmed resolvable --
`xlings install mcpp` in a clean sandbox installs it and builds a project
against a published index package.

A green CI round on the bumped pin is what makes this worth doing: it is direct
proof that the new release can build mcpp itself on every platform, which no
other check states.

Author: speak-agent

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "2026.9.8.1"
+    "mcpp": "2026.9.9.1"
   }
 }