ci: bootstrap pin -> 2026.7.27.1 (released, mirrored, indexed) (#294)

The separate post-release commit the pin guard now enforces: .xlings.json
and MCPP_PIN name a mcpp that already exists in xim-pkgindex, so they can
only move once the release has landed there.

Verified before bumping:
  - four-platform release + GitCode mirror 8/8 byte-identical (cmp against
    locally downloaded assets, not the upload exit code)
  - xim-pkgindex#429 merged, its four sha256 values recomputed locally and
    matching
  - clean XLINGS_HOME: xlings install mcpp@2026.7.27.1 resolves the
    four-segment version key, mcpp --version reports 2026.7.27.1, and a
    bare-name dependency (eigen = "5.0.1") resolves, builds and runs

Author: Sunrisepeak

.github/workflows/ci-fresh-install.yml

@@ -40,7 +40,7 @@ concurrency:
 # fails with `version not found` instead of silently testing an older
 # binary. Bump together with the .xlings.json workspace pin at release.
 env:
-  MCPP_PIN: '0.0.109'
+  MCPP_PIN: '2026.7.27.1'
 
 jobs:
   # ──────────────────────────────────────────────────────────────────

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "0.0.109"
+    "mcpp": "2026.7.27.1"
   }
 }