Add dlogf printf formatting, ipv4::to_string helper, clean up string building

firmware/lib/dispatch.cpp

@@ -72,7 +72,11 @@ void dispatch_schedule_ms(uint32_t ms, std::function<void()> fn) {
                     uint8_t err_buf[256];
                     span_writer err_out(err_buf, sizeof(err_buf));
                     auto err = encode_response_into(err_out, msg->message_id,
-                        DeviceError{2, "response too large: " + std::to_string(data.size())});
+                        [&]{
+                            char m[48];
+                            snprintf(m, sizeof(m), "response too large: %zu", data.size());
+                            return DeviceError{2, m};
+                        }());
                     if (err) usb.send(std::span<const uint8_t>{err_buf, *err});
                 }
             });