Fix #594: Map keys become dangling references when pushed into Vector (#650)

The Handle_Return_Ref specialization for const references was wrapping
return values in std::cref() while marking them as return values
(is_return_value=true). This caused Vector.push_back() to store the
reference directly without cloning, since it assumes return values are
freshly created temporaries. When the source object (e.g., a map) went
out of scope, the vector contained dangling references to freed memory.

The fix sets is_return_value=false for const reference returns, which
correctly triggers push_back to clone the value instead of storing a
bare reference. This is consistent with the non-const reference handler
(Handle_Return<Ret &>) which also does not set the return value flag.

Co-authored-by: leftibot <leftibot@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

include/chaiscript/dispatchkit/handle_return.hpp

@@ -126,7 +126,7 @@ namespace chaiscript {
       struct Handle_Return_Ref {
         template<typename T>
         static Boxed_Value handle(T &&r) {
-          return Boxed_Value(std::cref(r), true);
+          return Boxed_Value(std::cref(r), false);
         }
       };
 

unittests/map_keys_vector_push_back.chai

@@ -0,0 +1,29 @@
+// Regression test for issue #594
+// Map keys pushed into a vector via .first should remain valid
+// after the map goes out of scope.
+
+def keys(Map map)
+{
+  var v = Vector();
+  for( i : map )
+  {
+    v.push_back(i.first);
+  }
+  return v;
+}
+
+var k = Vector();
+if ( true )
+{
+  var m = ["a":"x", "b":"y", "c":"z"];
+  k = keys(m);
+}
+
+// After the map is out of scope, the keys should still be valid strings
+assert_equal(3, k.size())
+
+// Verify each element is a non-empty string
+for (elem : k)
+{
+  assert_true(elem.size() > 0)
+}