fix: Windows std module build — restore shq outer quotes + absolute paths

shq() needs outer double quotes for arguments with spaces (like
MSVC's std.ixx path in "Program Files"). Restored outer quotes in
shq() with a note: don't use shq for the first token (binary path).

std_module_build_commands on Windows uses absolute paths and raw
binary path as first token to avoid cmd.exe quote-stripping.

Author: Sunrisepeak

src/toolchain/clang.cppm

@@ -194,6 +194,27 @@ std::vector<std::string> std_module_build_commands(const Toolchain& tc,
                                                    const std::filesystem::path& bmiPath,
                                                    std::string_view sysrootFlag) {
     auto relBmi = std::filesystem::relative(bmiPath, cacheDir).string();
+#if defined(_WIN32)
+    // Windows: use absolute paths, raw binary path as first token
+    // (cmd.exe strips leading quotes), shq for args with spaces.
+    auto absBmi = (cacheDir / relBmi).string();
+    return {
+        std::format(
+            "{} -std=c++23 -Wno-reserved-module-identifier{} "
+            "--precompile {} -o {}",
+            tc.binaryPath.string(),
+            sysrootFlag,
+            mcpp::xlings::shq(tc.stdModuleSource.string()),
+            mcpp::xlings::shq(absBmi)),
+        std::format(
+            "{} -std=c++23 -Wno-reserved-module-identifier{} "
+            "{} -c -o {}",
+            tc.binaryPath.string(),
+            sysrootFlag,
+            mcpp::xlings::shq(absBmi),
+            mcpp::xlings::shq((cacheDir / "std.o").string()))
+    };
+#else
     return {
         std::format(
             "cd {} && {}{} -std=c++23 -Wno-reserved-module-identifier{} "
@@ -213,6 +234,7 @@ std::vector<std::string> std_module_build_commands(const Toolchain& tc,
             sysrootFlag,
             mcpp::xlings::shq(relBmi))
     };
+#endif
 }
 
 std::filesystem::path archive_tool(const Toolchain& tc) {

src/xlings.cppm

@@ -321,15 +321,16 @@ std::string shq(std::string_view s) {
     std::string out;
     out.reserve(s.size() + 2);
 #if defined(_WIN32)
-    // Windows: popen/system go through cmd.exe /c. To avoid cmd.exe's
-    // special quote-stripping when the command starts with ", we don't
-    // wrap in outer quotes. Instead, escape inner " as \" which the
-    // MSVC C runtime's argv parser understands.
+    // Windows: wrap in double quotes, escape inner " as \".
+    // IMPORTANT: avoid placing a shq'd token as the FIRST token in a
+    // popen/system command — cmd.exe strips a leading " pair.  For
+    // binary paths, use the raw string; shq is safe for arguments.
+    out.push_back('"');
     for (char c : s) {
         if (c == '"') out += "\\\"";
         else out.push_back(c);
     }
-    return out;
+    out.push_back('"');
 #else
     out.push_back('\'');
     for (char c : s) {