ShellQuote: use double quotes on Windows

cmd.exe treats single quotes as literal characters, so the existing
single-quote wrapping broke git/cmake invocations on a Windows host
("could not create leading directories of '<path>'").

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

implementations/Crafter.Build-External.cpp

@@ -37,6 +37,18 @@ std::string DeriveName(const GitSource& source) {
 }
 
 std::string ShellQuote(std::string_view s) {
+    // Windows cmd treats single quotes as literal characters — only double
+    // quotes work. Paths don't contain " in practice, so a simple wrap
+    // suffices; backslashes inside the path stay backslashes.
+#ifdef _WIN32
+    std::string out = "\"";
+    for (char c : s) {
+        if (c == '"') out += "\\\"";
+        else out += c;
+    }
+    out += "\"";
+    return out;
+#else
     std::string out = "'";
     for (char c : s) {
         if (c == '\'') out += "'\\''";
@@ -44,6 +56,7 @@ std::string ShellQuote(std::string_view s) {
     }
     out += "'";
     return out;
+#endif
 }
 
 std::string JoinOptions(std::span<const std::string> options) {