Add unit test framework and test stubs

- Add test/test_kiss.cpp: Unit tests for KISS protocol encoder/decoder
  Tests: frame decoding with/without escape sequences, port extraction,
  round-trip encoding/decoding, signal quality encoding, buffer overflow handling
- Add test/test_commands.cpp: Unit tests for config command parsing
  Tests: big-endian encoding/decoding, frequency frame parsing, type byte decoding
- Configure PlatformIO native test environment with GoogleTest framework
- Tests currently build but require linking stubs for full integration

Note: Full end-to-end testing requires mocking Serial I/O and radio functions,
which would be handled by integration tests on actual hardware or with a
more sophisticated test harness (e.g., CMake + GoogleTest).

platformio.ini

@@ -58,3 +58,14 @@ build_flags =
     -DLORA_SYNCWORD=0x12
     ; KISS serial baud rate
     -DKISS_BAUD=115200
+
+; ------------------------------------------------------------------
+; Native test environment — run unit tests on host
+; ------------------------------------------------------------------
+[env:test]
+platform = native
+test_framework = googletest
+test_build_src = yes
+build_flags =
+    -std=c99
+build_src_filter = +<*> -<main.cpp> -<radio.cpp>