Test against the released engine

mcpp 2026.8.27.1 is published and the index's latest points at it, so the pin
moves to it and the repository variable MCPP_SOURCE_REF is cleared. What this
repository tests is now the engine a user installs rather than a branch of it.

The manifests are unchanged: they already name the versions this round
publishes, and this repository's continuous integration substitutes working
trees for them, so the change spanning these repositories is still tested as
written.

Author: speak-agent

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ on:
 
 env:
   MCPP_SOURCE_REF: ${{ github.event.inputs.mcpp_ref || vars.MCPP_SOURCE_REF }}
-  MCPP_VERSION: 2026.8.26.2
+  MCPP_VERSION: 2026.8.27.1
   XLINGS_VERSION: v2026.8.17.2
   XLINGS_NON_INTERACTIVE: '1'