fix: mcpp auto-resolves xpkg from global xlings, remove all CI workarounds

When xlings installs a toolchain, the payload may land in xlings'
global data dir instead of mcpp's sandbox. package_fetcher now checks
~/.xlings/data/xpkgs/ as fallback and copies automatically.

Removed from CI/release:
- xlings install llvm (mcpp handles it)
- Pre-seed cp -r of LLVM xpkgs (mcpp handles it)

mcpp now works like a real user: just `xlings install mcpp` then `mcpp build`.

Author: Sunrisepeak

.github/workflows/ci-windows.yml

@@ -56,8 +56,7 @@ jobs:
           export PATH="$USERPROFILE/.xlings/subos/default/bin:$PATH"
           echo "$USERPROFILE/.xlings/subos/default/bin" >> "$GITHUB_PATH"
           xlings.exe --version
-          xlings.exe install llvm -y || xlings.exe install llvm@20.1.7 -y
-          xlings.exe install mcpp -y || xlings.exe install mcpp@0.0.17 -y
+          xlings.exe install mcpp -y
           echo "=== Searching for mcpp binary ==="
           find "$USERPROFILE/.xlings" -name "mcpp.exe" -o -name "mcpp" 2>/dev/null | head -10
           MCPP=$(find "$USERPROFILE/.xlings" -name "mcpp.exe" -path "*/bin/*" 2>/dev/null | head -1)
@@ -76,16 +75,6 @@ jobs:
         run: |
           export MCPP_VENDORED_XLINGS="$XLINGS_BIN"
 
-          # Pre-seed mcpp sandbox with xlings LLVM (avoids redundant 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"
-            rm -rf "$MCPP_XPKGS/xim-x-llvm"
-            cp -r "$XLINGS_XPKGS/xim-x-llvm" "$MCPP_XPKGS/xim-x-llvm"
-            echo "Pre-seeded LLVM from global xlings"
-          fi
-
           "$MCPP" build
 
           MCPP_SELF=$(find target -name "mcpp.exe" -path "*/bin/*" | head -1)

.github/workflows/release.yml

@@ -462,8 +462,7 @@ jobs:
           export PATH="$USERPROFILE/.xlings/subos/default/bin:$PATH"
           echo "$USERPROFILE/.xlings/subos/default/bin" >> "$GITHUB_PATH"
           xlings.exe --version
-          xlings.exe install llvm -y || xlings.exe install llvm@20.1.7 -y
-          xlings.exe install mcpp -y || xlings.exe install mcpp@0.0.17 -y
+          xlings.exe install mcpp -y
           MCPP=$(find "$USERPROFILE/.xlings" -name "mcpp.exe" -path "*/bin/*" 2>/dev/null | head -1)
           if [ -z "$MCPP" ]; then
             MCPP=$(find "$USERPROFILE/.xlings" -name "mcpp" -path "*/bin/*" 2>/dev/null | head -1)
@@ -481,15 +480,6 @@ jobs:
         run: |
           export MCPP_VENDORED_XLINGS="$XLINGS_BIN"
 
-          # Pre-seed mcpp sandbox with xlings LLVM (avoids redundant download)
-          MCPP_XPKGS="$USERPROFILE/.mcpp/registry/data/xpkgs"
-          if [ -d "$XLINGS_XPKGS/xim-x-llvm" ]; then
-            mkdir -p "$MCPP_XPKGS"
-            rm -rf "$MCPP_XPKGS/xim-x-llvm"
-            cp -r "$XLINGS_XPKGS/xim-x-llvm" "$MCPP_XPKGS/xim-x-llvm"
-            echo "Pre-seeded LLVM from global xlings"
-          fi
-
           "$MCPP" build
 
           MCPP_BIN=$(find target -name "mcpp.exe" -path "*/bin/*" | head -1)

src/pm/package_fetcher.cppm

@@ -605,6 +605,28 @@ Fetcher::resolve_xpkg_path(std::string_view target,
     };
 
     auto resolve = [&]() -> std::expected<XpkgPayload, CallError> {
+        // xlings may install the package into its global home rather than
+        // the mcpp sandbox. If the expected path is missing, copy from the
+        // global xlings data directory.
+        if (!std::filesystem::exists(verdir)) {
+            auto xhome = std::getenv("HOME");
+#if defined(_WIN32)
+            if (!xhome) xhome = std::getenv("USERPROFILE");
+#endif
+            if (xhome) {
+                auto globalDir = std::filesystem::path(xhome)
+                    / ".xlings" / "data" / "xpkgs"
+                    / verdir.parent_path().filename()
+                    / verdir.filename();
+                std::error_code ec;
+                if (std::filesystem::exists(globalDir, ec)) {
+                    std::filesystem::create_directories(verdir.parent_path(), ec);
+                    std::filesystem::copy(globalDir, verdir,
+                        std::filesystem::copy_options::recursive
+                        | std::filesystem::copy_options::overwrite_existing, ec);
+                }
+            }
+        }
         if (!std::filesystem::exists(verdir)) {
             return std::unexpected(CallError{
                 std::format("xpkg payload missing: {}", verdir.string())});