fix: Windows xlings command execution — use _putenv_s instead of cmd.exe set

cmd.exe `set` in compound &&-chains is unreliable. Instead, set
XLINGS_HOME/XLINGS_PROJECT_DIR/PATH directly in the process
environment via _putenv_s (inherited by popen/system children).

Also:
- Skip patchelf bootstrap on Windows (ELF-only, like macOS)
- Create sandbox home dir before xlings self init

Author: Sunrisepeak

src/config.cppm

@@ -564,8 +564,8 @@ std::expected<GlobalConfig, ConfigError> load_or_init(
     //    upstream (see docs/short-term-vs-long-track plan).
     ensure_sandbox_xlings_binary(cfg, quiet);
     ensure_sandbox_init(cfg, quiet);
-#if !defined(__APPLE__)
-    // patchelf is ELF-only; macOS uses Mach-O and does not need it.
+#if !defined(__APPLE__) && !defined(_WIN32)
+    // patchelf is ELF-only; macOS uses Mach-O and Windows uses PE.
     ensure_sandbox_patchelf(cfg, quiet, onBootstrapProgress);
 #endif
     ensure_sandbox_ninja(cfg, quiet, onBootstrapProgress);

src/xlings.cppm

@@ -11,6 +11,7 @@ module;
 #include <cstdio>
 #include <cstdlib>
 #if defined(_WIN32)
+#include <stdlib.h>    // _putenv_s
 #define popen  _popen
 #define pclose _pclose
 #endif
@@ -424,22 +425,18 @@ std::filesystem::path sandbox_init_marker(const Env& env) {
 std::string build_command_prefix(const Env& env) {
     auto xvmBin = paths::sandbox_bin(env).string();
 #if defined(_WIN32)
-    // Windows: use cmd.exe set + call semantics.
-    if (env.projectDir.empty()) {
-        return std::format(
-            "cd /d {} && set \"PATH={};%PATH%\" && set \"XLINGS_HOME={}\" && set \"XLINGS_PROJECT_DIR=\" && {}",
-            shq(env.home.string()),
-            xvmBin,
-            env.home.string(),
-            shq(env.binary.string()));
+    // Windows: set environment variables via the process environment
+    // (cmd.exe `set` in compound &&-chains is unreliable) then invoke
+    // xlings directly. _putenv_s is inherited by popen/system child.
+    _putenv_s("XLINGS_HOME", env.home.string().c_str());
+    _putenv_s("XLINGS_PROJECT_DIR",
+              env.projectDir.empty() ? "" : env.projectDir.string().c_str());
+    // Prepend sandbox bin to PATH
+    {
+        std::string newPath = xvmBin + ";" + (std::getenv("PATH") ? std::getenv("PATH") : "");
+        _putenv_s("PATH", newPath.c_str());
     }
-    return std::format(
-        "cd /d {} && set \"PATH={};%PATH%\" && set \"XLINGS_HOME={}\" && set \"XLINGS_PROJECT_DIR={}\" && {}",
-        shq(env.home.string()),
-        xvmBin,
-        env.home.string(),
-        env.projectDir.string(),
-        shq(env.binary.string()));
+    return shq(env.binary.string());
 #else
     if (env.projectDir.empty()) {
         // Global mode: unset XLINGS_PROJECT_DIR (existing behavior).
@@ -663,10 +660,9 @@ int install_with_progress(const Env& env, std::string_view target,
         R"({{"targets":["{}"],"yes":true}})", target);
 
 #if defined(_WIN32)
-    auto cmd = std::format(
-        "cd /d {} && set \"XLINGS_PROJECT_DIR=\" && set \"XLINGS_HOME={}\" && {} interface install_packages --args {} 2>nul",
-        shq(env.home.string()),
-        env.home.string(),
+    _putenv_s("XLINGS_HOME", env.home.string().c_str());
+    _putenv_s("XLINGS_PROJECT_DIR", "");
+    auto cmd = std::format("{} interface install_packages --args {} 2>nul",
         shq(env.binary.string()),
         shq(argsJson));
 #else
@@ -787,13 +783,16 @@ void ensure_init(const Env& env, bool quiet) {
     auto marker = paths::sandbox_init_marker(env);
     if (std::filesystem::exists(marker)) return;
 
+    // Ensure the home directory exists before cd'ing into it.
+    std::error_code ec;
+    std::filesystem::create_directories(env.home, ec);
+
     if (!quiet)
         print_status("Initialize", "mcpp sandbox layout (one-time)");
 #if defined(_WIN32)
-    auto cmd = std::format(
-        "cd /d {} && set \"XLINGS_PROJECT_DIR=\" && set \"XLINGS_HOME={}\" && {} self init >nul 2>&1",
-        shq(env.home.string()),
-        env.home.string(),
+    _putenv_s("XLINGS_HOME", env.home.string().c_str());
+    _putenv_s("XLINGS_PROJECT_DIR", "");
+    auto cmd = std::format("{} self init >nul 2>&1",
         shq(env.binary.string()));
 #else
     auto cmd = std::format(