fix: reuse macos ci llvm toolchain

Author: Sunrisepeak

.github/workflows/ci-macos.yml

@@ -19,6 +19,10 @@ jobs:
     name: macOS ARM64 — xlings LLVM end-to-end
     runs-on: macos-15
     timeout-minutes: 30
+    env:
+      # Match dev mcpp's default home so toolchains installed for CI are
+      # visible to the freshly-built target/.../bin/mcpp used below.
+      MCPP_HOME: /Users/runner/.mcpp
     steps:
       - uses: actions/checkout@v4
 
@@ -64,6 +68,17 @@ jobs:
           echo "LLVM_ROOT=$LLVM_ROOT" >> "$GITHUB_ENV"
           echo "CXX=$LLVM_ROOT/bin/clang++" >> "$GITHUB_ENV"
 
+      - name: Prime mcpp sandbox with LLVM
+        run: |
+          SRC="$HOME/.xlings/data/xpkgs/xim-x-llvm/20.1.7"
+          DST="$MCPP_HOME/registry/data/xpkgs/xim-x-llvm/20.1.7"
+          test -d "$SRC"
+          rm -rf "$DST"
+          mkdir -p "$(dirname "$DST")"
+          cp -R "$SRC" "$DST"
+          printf '1\n' > "$DST/.mcpp_ok"
+          ls "$DST/bin/clang++"
+
       - name: Inspect LLVM package structure
         run: |
           echo "=== bin/ ==="