recover system_state requirement on random_access_iterator

Author: WenyinWei

include/core/concepts.hpp

@@ -1,6 +1,7 @@
 #pragma once
 #include <concepts>
 #include <type_traits>
+#include <iterator>
 #include <string>
 
 // Time type concept - basic arithmetic types that can represent time
@@ -13,8 +14,10 @@ concept system_state = requires(T state) {
     typename T::value_type;
     requires std::is_arithmetic_v<typename T::value_type>;
     requires !std::same_as<T, std::string>; // Exclude string types
-    { state.size() } -> std::convertible_to<std::size_t>;
-    { state.begin() };
-    { state.end() };
+    requires requires {
+        { state.size() } -> std::convertible_to<std::size_t>;
+        { state.begin() } -> std::random_access_iterator;
+        { state.end() } -> std::random_access_iterator;
+    };
     { state[0] } -> std::convertible_to<typename T::value_type>;
 };