feat(companion_radio): add external FEM gain preferences for RX and TX for companions

Introduced consistent preferences for external LoRa FEM RX and TX gain settings in NodePrefs. Updated companion MyMesh to apply these settings during initialization and transmission. Added unit tests to verify the round-trip serialization of these new preferences.

test/test_companion_node_prefs/test_companion_node_prefs.cpp

@@ -0,0 +1,80 @@
+#include <gtest/gtest.h>
+
+#include <cstdio>
+#include <cstring>
+#include <string>
+
+#include "../../examples/companion_radio/NodePrefs.h"
+
+class ReplayStream : public Stream {
+  const char* _text;
+  int _pos = 0;
+  int _len;
+
+public:
+  explicit ReplayStream(const char* text) : _text(text), _len(strlen(text)) { }
+
+  int available() override { return _len - _pos; }
+  int read() override { return _pos < _len ? _text[_pos++] : -1; }
+  int peek() override { return _pos < _len ? _text[_pos] : -1; }
+};
+
+class CaptureStream : public Stream {
+  std::string _text;
+
+  size_t emit(long long value) {
+    char text[24];
+    int length = snprintf(text, sizeof(text), "%lld", value);
+    return write(reinterpret_cast<const uint8_t*>(text), length);
+  }
+
+public:
+  size_t write(uint8_t value) override {
+    _text.push_back(static_cast<char>(value));
+    return 1;
+  }
+
+  size_t write(const uint8_t* buffer, size_t size) override {
+    _text.append(reinterpret_cast<const char*>(buffer), size);
+    return size;
+  }
+
+  size_t print(unsigned char value, int = DEC) override { return emit(value); }
+  size_t print(int value, int = DEC) override { return emit(value); }
+  size_t print(unsigned int value, int = DEC) override { return emit(value); }
+  size_t print(long value, int = DEC) override { return emit(value); }
+  size_t print(unsigned long value, int = DEC) override { return emit(value); }
+  size_t print(long long value, int = DEC) override { return emit(value); }
+  size_t print(unsigned long long value, int = DEC) override { return emit(value); }
+
+  const std::string& text() const { return _text; }
+};
+
+TEST(CompanionNodePrefs, RxGainSettingsRoundTripIndependently) {
+  NodePrefs saved;
+  saved.rx_boosted_gain = 0;
+  saved.radio_fem_rxgain = 1;
+  saved.radio_fem_txgain = 0;
+
+  CaptureStream output;
+  ASSERT_TRUE(saved.saveSerial(output));
+  EXPECT_NE(std::string::npos, output.text().find("rxgain:0"));
+  EXPECT_NE(std::string::npos, output.text().find("fem_rxgain:1"));
+  EXPECT_NE(std::string::npos, output.text().find("fem_txgain:0"));
+
+  ReplayStream input("{radio:{rxgain:1,fem_rxgain:0,fem_txgain:1}}");
+  NodePrefs loaded;
+  loaded.rx_boosted_gain = 0;
+  loaded.radio_fem_rxgain = 1;
+  loaded.radio_fem_txgain = 0;
+
+  ASSERT_TRUE(loaded.loadSerial(input));
+  EXPECT_EQ(1, loaded.rx_boosted_gain);
+  EXPECT_EQ(0, loaded.radio_fem_rxgain);
+  EXPECT_EQ(1, loaded.radio_fem_txgain);
+}
+
+int main(int argc, char** argv) {
+  ::testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+}