fix: use system xlings with XLINGS_HOME to install LLVM into mcpp sandbox

Author: Sunrisepeak

.github/workflows/ci-windows.yml

@@ -56,10 +56,6 @@ jobs:
           export PATH="$USERPROFILE/.xlings/subos/default/bin:$PATH"
           echo "$USERPROFILE/.xlings/subos/default/bin" >> "$GITHUB_PATH"
           xlings.exe --version
-          # Install LLVM first via system xlings — mcpp's sandboxed xlings
-          # has an upstream bug where large package extraction fails on Windows.
-          # See: https://github.com/mcpp-community/mcpp/pull/53
-          xlings.exe install llvm -y || xlings.exe install llvm@20.1.7 -y
           xlings.exe install mcpp -y
           echo "=== Searching for mcpp binary ==="
           find "$USERPROFILE/.xlings" -name "mcpp.exe" -o -name "mcpp" 2>/dev/null | head -10
@@ -79,19 +75,21 @@ jobs:
         run: |
           export MCPP_VENDORED_XLINGS="$XLINGS_BIN"
 
-          # Find where xlings installed LLVM and pre-seed into mcpp sandbox
-          MCPP_XPKGS="$USERPROFILE/.mcpp/registry/data/xpkgs"
-          LLVM_SRC=$(find "$USERPROFILE/.xlings" -path "*/xim-x-llvm/20.1.7/bin/clang++.exe" 2>/dev/null | head -1)
-          if [ -n "$LLVM_SRC" ]; then
-            # Go up from bin/clang++.exe -> 20.1.7 -> xim-x-llvm
-            LLVM_VER_DIR=$(dirname "$(dirname "$LLVM_SRC")")
-            LLVM_PKG_DIR=$(dirname "$LLVM_VER_DIR")
-            mkdir -p "$MCPP_XPKGS"
-            cp -r "$LLVM_PKG_DIR" "$MCPP_XPKGS/xim-x-llvm"
-            echo "Pre-seeded LLVM from $LLVM_PKG_DIR"
-          else
-            echo "WARNING: LLVM not found in xlings after install"
-            find "$USERPROFILE/.xlings" -name "clang++.exe" 2>/dev/null | head -5
+          # Install LLVM directly into mcpp sandbox via xlings.
+          # xlings sandboxed install has issues with large packages on Windows,
+          # so we use the system xlings binary with XLINGS_HOME pointed at
+          # the mcpp sandbox to ensure LLVM lands in the right place.
+          XLINGS_SYS="$USERPROFILE/.xlings/subos/default/bin/xlings.exe"
+          MCPP_REGISTRY="$USERPROFILE/.mcpp/registry"
+          export XLINGS_HOME="$MCPP_REGISTRY"
+          "$XLINGS_SYS" install llvm -y || "$XLINGS_SYS" install llvm@20.1.7 -y
+          unset XLINGS_HOME
+
+          # Verify LLVM is in the mcpp sandbox
+          LLVM_CHECK="$MCPP_REGISTRY/data/xpkgs/xim-x-llvm"
+          if [ ! -d "$LLVM_CHECK" ]; then
+            echo "LLVM not in sandbox, searching..."
+            find "$USERPROFILE" -path "*/xim-x-llvm/20.1.7/bin/clang++.exe" 2>/dev/null | head -3
           fi
 
           "$MCPP" build