Add flow control to the companion USB interface

The contact sync streams up to MAX_CONTACTS frames back to back, but
ArduinoSerialInterface never checked whether the stream could take
them: isWriteBusy() returned false unconditionally, so MyMesh's pacing
gate had no effect, and writeFrame() called write() without looking at
availableForWrite(). On ESP32 (HWCDC, 256 byte TX ring) a host that
stalls for a moment makes the driver drop queued bytes silently, which
tears a frame in half - and since the framing is length prefixed with
no checksum and no resync marker, the client stays desynchronised for
the rest of the session. Reported as 'the app disconnects while
syncing contacts' on devices with a large contact list.

Add opt-in flow control: report busy until a whole frame fits, and
drop frames as a unit instead of tearing them. Enable it for the
companion USB interface and give HWCDC a bigger TX buffer with a short
write timeout, mirroring what kiss_modem already does.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

examples/companion_radio/main.cpp

@@ -218,7 +218,14 @@ void setup() {
 // add usb interface
 #if defined(ENABLE_USB_INTERFACE)
   usb_serial_interface.begin(Serial);
+  // keep frames intact and pace the contact stream when the host is slow
+  usb_serial_interface.enableFlowControl(true);
 #if defined(ESP32) && defined(ARDUINO_USB_MODE) && ARDUINO_USB_MODE == 1
+  // a 256 byte TX buffer overflows during a contact sync, and write() blocks
+  // up to tx_timeout_ms per call against a stalled host (same reasoning as
+  // the kiss_modem tuning)
+  Serial.setTxBufferSize(4096);
+  Serial.setTxTimeoutMs(5);
   // The ESP32 USB-Serial-JTAG peripheral (HWCDC) has no DTR concept at all:
   // (bool)Serial only tells us the host has enumerated the device, which is
   // already true when the cable is plugged into a powered port. Fall back to