Replace the implementation of cxper_char_traits::move() with a simpler one

Reason: See the new code comment.

test/constexpr_tests.hpp

@@ -69,12 +69,16 @@ struct cxper_char_traits
 
   static constexpr char_type* move(char_type* dest, const char_type* src, std::size_t n)
   {
-    if (detail::ptr_in_range(src, src + n, dest))
-    {
-      while (n--)
-        assign(dest[n], src[n]);
-      return dest;
-    }
+    // This implementation does not handle overlapping ranges where
+    // dest > src. A correct implementation would need to detect this
+    // case and copy backwards, but detecting overlap requires pointer
+    // comparisons that are not allowed in constant expressions when
+    // the pointers point to unrelated objects (e.g., a string literal
+    // and the static_string's internal buffer).
+    //
+    // Since cxper_char_traits is only used for testing constexpr
+    // functionality and the tests do not exercise overlapping moves
+    // where dest > src, this simple forward copy is sufficient.
     return copy(dest, src, n);
   }