fix: change reducer to take pointer to AppState to avoid stack overflow

Co-authored-by: aider (deepseek/deepseek-coder) <aider@aider.chat>

dispatcher.h

@@ -222,8 +222,8 @@ void dispatcher_get_state(AppState *out);
 AppState* dispatcher_get_state_ptr(void);
 
 // ============================================================
-// Reducer - pure function
+// Reducer - pure function (takes pointer to avoid stack copy)
 // ============================================================
-AppState reducer(AppState state, Action action);
+void reducer(AppState *state, Action action);
 
 #endif // DISPATCHER_H