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).

test/test_kiss.cpp

@@ -0,0 +1,135 @@
+#include <gtest/gtest.h>
+#include <cstring>
+
+extern "C" {
+#include "kiss.h"
+}
+
+TEST(KissDecoder, InitState) {
+    kiss_decoder_t dec;
+    kiss_decoder_init(&dec);
+    EXPECT_EQ(dec.state, KISS_STATE_IDLE);
+    EXPECT_EQ(dec.len, 0);
+}
+
+TEST(KissDecoder, DecodeSimpleFrame) {
+    kiss_decoder_t dec;
+    kiss_decoder_init(&dec);
+    kiss_frame_t frame;
+
+    /* Build frame: FEND [0x00 0xAA 0xBB] FEND */
+    EXPECT_FALSE(kiss_decode(&dec, KISS_FEND, &frame));     /* FEND start */
+    EXPECT_FALSE(kiss_decode(&dec, 0x00, &frame));          /* type byte */
+    EXPECT_FALSE(kiss_decode(&dec, 0xAA, &frame));          /* data */
+    EXPECT_FALSE(kiss_decode(&dec, 0xBB, &frame));          /* data */
+    EXPECT_TRUE(kiss_decode(&dec, KISS_FEND, &frame));      /* FEND end */
+
+    EXPECT_EQ(frame.port, 0);
+    EXPECT_EQ(frame.len, 2);
+    EXPECT_EQ(frame.data[0], 0xAA);
+    EXPECT_EQ(frame.data[1], 0xBB);
+}
+
+TEST(KissDecoder, DecodeWithEscape) {
+    kiss_decoder_t dec;
+    kiss_decoder_init(&dec);
+    kiss_frame_t frame;
+
+    /* Frame with escaped FEND: FEND [0x00 FESC TFEND] FEND */
+    EXPECT_FALSE(kiss_decode(&dec, KISS_FEND, &frame));
+    EXPECT_FALSE(kiss_decode(&dec, 0x00, &frame));
+    EXPECT_FALSE(kiss_decode(&dec, KISS_FESC, &frame));
+    EXPECT_FALSE(kiss_decode(&dec, KISS_TFEND, &frame));
+    EXPECT_TRUE(kiss_decode(&dec, KISS_FEND, &frame));
+
+    EXPECT_EQ(frame.port, 0);
+    EXPECT_EQ(frame.len, 1);
+    EXPECT_EQ(frame.data[0], KISS_FEND);
+}
+
+TEST(KissDecoder, DecodePortExtraction) {
+    kiss_decoder_t dec;
+    kiss_decoder_init(&dec);
+    kiss_frame_t frame;
+
+    /* Frame with port 2: FEND [0x21 0xAA] FEND (port=2 in upper nibble) */
+    EXPECT_FALSE(kiss_decode(&dec, KISS_FEND, &frame));
+    EXPECT_FALSE(kiss_decode(&dec, 0x21, &frame));          /* port=2, cmd=1 */
+    EXPECT_FALSE(kiss_decode(&dec, 0xAA, &frame));
+    EXPECT_TRUE(kiss_decode(&dec, KISS_FEND, &frame));
+
+    EXPECT_EQ(frame.port, 2);
+    EXPECT_EQ(frame.len, 1);
+    EXPECT_EQ(frame.data[0], 0xAA);
+}
+
+TEST(KissEncoder, EncodeSimpleFrame) {
+    uint8_t data[] = {0xAA, 0xBB};
+    uint8_t dst[32];
+    size_t len = kiss_encode(0, data, 2, dst, sizeof(dst));
+
+    EXPECT_EQ(len, 6);  /* FEND type data[2] FEND = 6 bytes */
+    EXPECT_EQ(dst[0], KISS_FEND);
+    EXPECT_EQ(dst[1], 0x00);  /* type byte */
+    EXPECT_EQ(dst[2], 0xAA);
+    EXPECT_EQ(dst[3], 0xBB);
+    EXPECT_EQ(dst[4], KISS_FEND);
+}
+
+TEST(KissEncoder, EncodeWithEscape) {
+    uint8_t data[] = {KISS_FEND};
+    uint8_t dst[32];
+    size_t len = kiss_encode(0, data, 1, dst, sizeof(dst));
+
+    EXPECT_EQ(len, 7);  /* FEND type FESC TFEND FEND = 7 bytes */
+    EXPECT_EQ(dst[0], KISS_FEND);
+    EXPECT_EQ(dst[1], 0x00);
+    EXPECT_EQ(dst[2], KISS_FESC);
+    EXPECT_EQ(dst[3], KISS_TFEND);
+    EXPECT_EQ(dst[4], KISS_FEND);
+}
+
+TEST(KissEncoder, EncodeQuality) {
+    uint8_t dst[32];
+    size_t len = kiss_encode_quality(-7, -103, dst, sizeof(dst));
+
+    EXPECT_EQ(len, 8);  /* FEND type(0x11) snr rssi_hi rssi_lo FEND */
+    EXPECT_EQ(dst[0], KISS_FEND);
+    EXPECT_EQ(dst[1], 0x11);  /* port=1, cmd=1 */
+    EXPECT_EQ(dst[2], 0xF9);  /* -7 in two's complement */
+    EXPECT_EQ(dst[3], 0xFF);  /* -103 >> 8 */
+    EXPECT_EQ(dst[4], 0x99);  /* -103 & 0xFF */
+    EXPECT_EQ(dst[5], KISS_FEND);
+}
+
+TEST(KissEncoder, RoundTrip) {
+    uint8_t orig_data[] = {0x11, 0x22, KISS_FEND, KISS_FESC, 0x33};
+    uint8_t encoded[64];
+    uint8_t decoded_data[64];
+
+    size_t enc_len = kiss_encode(1, orig_data, 5, encoded, sizeof(encoded));
+    EXPECT_GT(enc_len, 0);
+
+    kiss_decoder_t dec;
+    kiss_decoder_init(&dec);
+    kiss_frame_t frame;
+
+    for (size_t i = 0; i < enc_len; i++) {
+        if (kiss_decode(&dec, encoded[i], &frame)) {
+            EXPECT_EQ(frame.port, 1);
+            EXPECT_EQ(frame.len, 5);
+            for (int j = 0; j < 5; j++) {
+                EXPECT_EQ(frame.data[j], orig_data[j]);
+            }
+            return;
+        }
+    }
+    FAIL() << "Decoder did not return complete frame";
+}
+
+TEST(KissEncoder, BufferOverflow) {
+    uint8_t data[] = {0xAA, 0xBB};
+    uint8_t dst[3];  /* Too small */
+    size_t len = kiss_encode(0, data, 2, dst, sizeof(dst));
+    EXPECT_EQ(len, 0);  /* Should return 0 on overflow */
+}