core: add observation for generator

lib/include/irritator/core.hpp

@@ -3343,11 +3343,15 @@ struct generator
     status lambda(
       data_array<output_port, output_port_id>& output_ports) noexcept
     {
-        if (auto* port = output_ports.try_to_get(y[0]); port)
-            port->messages.emplace_front(value);
+        output_ports.get(y[0]).messages.emplace_front(value);
 
         return status::success;
     }
+
+    message observation(time /*t*/) const noexcept
+    {
+        return message(value);
+    }
 };
 
 struct constant