fix: pre-seed mcpp sandbox with global xlings LLVM via junction

xlings install into the mcpp sandbox creates an empty stub for LLVM
(since it's already installed globally). Use a Windows directory
junction to link the mcpp sandbox's xim-x-llvm to the global one,
avoiding a redundant download and the empty-dir problem.

Author: Sunrisepeak

.github/workflows/ci-windows.yml

@@ -260,16 +260,23 @@ jobs:
           # Debug: test xlings directly
           "$USERPROFILE/.xlings/subos/default/bin/xlings.exe" --version || echo "xlings direct call failed"
 
-          # Debug: check what's in the mcpp sandbox after xlings installs
-          "$MCPP_EXE" build || {
-            echo "=== mcpp build failed, debugging ==="
-            echo "=== mcpp sandbox contents ==="
-            find "$USERPROFILE/.mcpp/registry/data/xpkgs" -name "*.exe" 2>/dev/null | head -20
-            echo "=== LLVM bin dir ==="
-            ls "$USERPROFILE/.mcpp/registry/data/xpkgs/xim-x-llvm/20.1.7/bin/" 2>/dev/null || echo "dir not found"
-            ls "$USERPROFILE/.mcpp/registry/data/xpkgs/xim-x-llvm/" 2>/dev/null || echo "llvm dir not found"
-            exit 1
-          }
+          # Pre-seed the mcpp sandbox with the already-installed LLVM from
+          # the global xlings. xlings won't re-download if the package dir
+          # exists, but it may create an empty stub. Symlinking avoids
+          # a redundant ~500MB download.
+          MCPP_XPKGS="$USERPROFILE/.mcpp/registry/data/xpkgs"
+          XLINGS_XPKGS="$USERPROFILE/.xlings/data/xpkgs"
+          if [ -d "$XLINGS_XPKGS/xim-x-llvm" ]; then
+            mkdir -p "$MCPP_XPKGS"
+            # Remove empty stub if mcpp's xlings created one
+            rm -rf "$MCPP_XPKGS/xim-x-llvm"
+            # Use junction (Windows symlink for directories)
+            cmd.exe //c "mklink /J \"$(cygpath -w "$MCPP_XPKGS/xim-x-llvm")\" \"$(cygpath -w "$XLINGS_XPKGS/xim-x-llvm")\""
+            echo "Linked LLVM xpkg from global xlings"
+            ls "$MCPP_XPKGS/xim-x-llvm/20.1.7/bin/" | head -5
+          fi
+
+          "$MCPP_EXE" build
 
           # Find the self-hosted binary
           SELF_MCPP=$(find target -name "mcpp.exe" -path "*/bin/*" | head -1)