bootstrap from 2026.9.17.1

2026.9.17.1 is released, mirrored to GitHub and GitCode (each archive compared
byte for byte), and indexed (openxlings/xim-pkgindex#854). The macOS 27
bootstrap wrapper in setup-macos-llvm retires with this pin.

Author: speak-agent

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "2026.9.16.1"
+    "mcpp": "2026.9.17.1"
   }
 }