Fix ambiguous MString << JString::Proxy overload resolution

JString::Proxy offers several implicit conversion operators (ZString,
MString, std::string, double, bool) so a single $[...]-style access can
be used wherever any of those types is expected. That flexibility made
"MString << Proxy" genuinely ambiguous: operator ZString() and
operator MString() (via its own implicit upcast to ZString) both satisfy
MString::operator<<(ZString), and operator double()/operator bool() both
satisfy MString::operator<<(int) via a standard numeric conversion, so
the compiler had no basis to prefer one conversion path over another.

Fixed the same way Proxy already solves this exact problem for
std::cout << proxy - a dedicated friend operator<<(MString&, const
Proxy&) that needs no conversion at all, so it's always the preferred
candidate. Verified by actually running testing/jstring_test.cpp (not
just compiling it) - the line that used to fail to compile now correctly
prints "this is a test: this is a comment".

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

JString.cpp

@@ -12,6 +12,13 @@ namespace coreutils {
       return os;
    }
 
+   MString &operator<<(MString &ms, const JString::Proxy &proxy) {
+      ZString tempkey = proxy.key;
+      ZString temp = proxy.jstring.find(tempkey);
+      ms << temp;
+      return ms;
+   }
+
    JString::JString() : MString("{}") {}
 
    // ---------------------------------------------------------------------