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.

platformio.ini

@@ -169,3 +169,18 @@ build_src_filter =
   +<../src/Packet.cpp>
 lib_deps =
   google/googletest @ 1.17.0
+
+[env:native_kiss_modem]
+platform = native
+test_framework = googletest
+build_flags = -std=c++17
+  -I test/mocks
+  -I src
+  -I examples/kiss_modem
+test_build_src = yes
+test_filter = test_kiss_modem
+build_src_filter =
+  -<*>
+  +<../examples/kiss_modem/KissModem.cpp>
+lib_deps =
+  google/googletest @ 1.17.0