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.

variants/heltec_v4/platformio.ini

@@ -432,5 +432,10 @@ lib_deps =
 
 [env:heltec_v4_kiss_modem]
 extends = Heltec_lora32_v4
+build_unflags =
+  -DARDUINO_USB_MODE=0
+build_flags =
+  ${Heltec_lora32_v4.build_flags}
+  -DARDUINO_USB_MODE=1
 build_src_filter = ${Heltec_lora32_v4.build_src_filter}
   +<../examples/kiss_modem/>

variants/station_g2/platformio.ini

@@ -241,5 +241,10 @@ lib_deps =
 
 [env:Station_G2_kiss_modem]
 extends = Station_G2
+build_unflags =
+  -DARDUINO_USB_MODE=0
+build_flags =
+  ${Station_G2.build_flags}
+  -DARDUINO_USB_MODE=1
 build_src_filter = ${Station_G2.build_src_filter}
   +<../examples/kiss_modem/>