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/Mesh.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#include <cstdint>
+
+namespace mesh {
+
+class Radio {
+public:
+  virtual ~Radio() = default;
+  virtual bool isReceiving() { return false; }
+  virtual uint32_t getEstAirtimeFor(uint16_t) { return 10; }
+  virtual bool startSendRaw(const uint8_t*, uint16_t) { return true; }
+  virtual bool isSendComplete() { return true; }
+  virtual void onSendFinished() {}
+  virtual int16_t getNoiseFloor() { return -120; }
+};
+
+class MainBoard {
+public:
+  virtual ~MainBoard() = default;
+  virtual uint16_t getBattMilliVolts() { return 4200; }
+  virtual float getMCUTemperature() { return 25.0f; }
+  virtual const char* getManufacturerName() { return "mock-board"; }
+  virtual void reboot() {}
+};
+
+}