fix: prevent ESP32 KISS modem stalls under USB backpressure

Switch ESP32-S3 KISS modem environments to HWCDC and move outbound KISS writes to a non-blocking queued frame path so loop() and TX completion keep progressing when the host reads slowly. Add native backpressure regression tests and correct the native_kiss_modem test filter so this suite runs directly with pio test.

test/mocks/Arduino.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <cstdint>
+#include <cmath>
+#include "Stream.h"
+
+inline uint32_t g_mock_millis = 0;
+
+using std::isnan;
+
+inline uint32_t millis() {
+  return g_mock_millis;
+}
+
+inline void delay(uint32_t ms) {
+  g_mock_millis += ms;
+}