feat(radio): drive LoRa FEM RX gain from radio_fem_rxgain (heltec_v4)

Completes the FEM RX-gain restoration begun in the CAD/prefs change, which
persisted radio_fem_rxgain but didn't yet drive the hardware. Also dropped
by the 22eb9b87 revert; restored to match upstream.

- MainBoard: setLoRaFemLnaEnabled()/canControlLoRaFemLna()/isLoRaFemLnaEnabled()
  virtuals (default: can't control — non-FEM boards report unsupported)
- heltec_v4: board overrides driving loRaFEMControl; LoRaFEMControl gains the
  isLNAEnabled() getter (it already tracked lna_enabled and drove the FEM)
- CLI: `set radio.fem.rxgain on/off` / `get radio.fem.rxgain` (guarded by
  canControlLoRaFemLna, so it reports "unsupported" on non-FEM boards)
- app startup applies the persisted pref: board.setLoRaFemLnaEnabled(
  _prefs.radio_fem_rxgain), beside setRxBoostedGainMode

Default is ON (upstream), so on FEM boards the LNA is enabled after upgrade —
a real reception behavior change to confirm on hardware. The other FEM
variants (heltec_t096/tower_v2/tracker_v2) need the same small board-override
addition; until then `radio.fem.rxgain` reports unsupported there (no
regression — status quo).

Builds: heltec_v4 repeater-observer + room-observer (FEM board), Heltec_v3
repeater (non-FEM, base virtuals no-op). NEEDS on-device validation on a
Heltec V4.

examples/companion_radio/MyMesh.cpp

@@ -970,6 +970,7 @@ void MyMesh::begin(bool has_display) {
   radio_driver.setRxBoostedGainMode(_prefs.rx_boosted_gain);
   MESH_DEBUG_PRINTLN("RX Boosted Gain Mode: %s",
                      radio_driver.getRxBoostedGainMode() ? "Enabled" : "Disabled");
+  board.setLoRaFemLnaEnabled(_prefs.radio_fem_rxgain);   // LoRa FEM LNA (FEM boards only)
 }
 
 const char *MyMesh::getNodeName() {

examples/simple_repeater/MyMesh.cpp

@@ -1073,6 +1073,7 @@ void MyMesh::begin(FILESYSTEM *fs) {
   radio_driver.setRxBoostedGainMode(_prefs.rx_boosted_gain);
   MESH_DEBUG_PRINTLN("RX Boosted Gain Mode: %s",
                      radio_driver.getRxBoostedGainMode() ? "Enabled" : "Disabled");
+  board.setLoRaFemLnaEnabled(_prefs.radio_fem_rxgain);   // LoRa FEM LNA (FEM boards only)
 
   updateAdvertTimer();
   updateFloodAdvertTimer();

examples/simple_room_server/MyMesh.cpp

@@ -735,6 +735,7 @@ void MyMesh::begin(FILESYSTEM *fs) {
 
   radio_driver.setParams(_prefs.freq, _prefs.bw, _prefs.sf, _prefs.cr);
   radio_driver.setTxPower(_prefs.tx_power_dbm);
+  board.setLoRaFemLnaEnabled(_prefs.radio_fem_rxgain);   // LoRa FEM LNA (FEM boards only)
 
   updateAdvertTimer();
   updateFloodAdvertTimer();

examples/simple_sensor/SensorMesh.cpp

@@ -772,6 +772,7 @@ void SensorMesh::begin(FILESYSTEM* fs) {
 
   radio_driver.setParams(_prefs.freq, _prefs.bw, _prefs.sf, _prefs.cr);
   radio_driver.setTxPower(_prefs.tx_power_dbm);
+  board.setLoRaFemLnaEnabled(_prefs.radio_fem_rxgain);   // LoRa FEM LNA (FEM boards only)
 
   updateAdvertTimer();
   updateFloodAdvertTimer();

src/MeshCore.h

@@ -69,6 +69,12 @@ public:
   // dry_run is true the build is only reported, not flashed. Observer (ESP32+WiFi) builds only.
   virtual bool otaFromManifest(const char* current_ver, bool dry_run, char reply[]) { return false; }
 
+  // LoRa front-end-module LNA (RX gain) control. Only FEM-equipped boards override
+  // these; others report they can't control it. Driven by NodePrefs.radio_fem_rxgain.
+  virtual bool setLoRaFemLnaEnabled(bool enable) { return false; }
+  virtual bool canControlLoRaFemLna() const { return false; }
+  virtual bool isLoRaFemLnaEnabled() const { return false; }
+
   // Power management interface (boards with power management override these)
   virtual bool isExternalPowered() { return false; }
   virtual uint16_t getBootVoltage() { return 0; }

src/helpers/CommonCLI.cpp

@@ -984,6 +984,28 @@ void CommonCLI::handleSetCmd(uint32_t sender_timestamp, char* command, char* rep
     _prefs->cad_enabled = memcmp(&config[4], "on", 2) == 0;
     savePrefs();
     strcpy(reply, "OK");
+  } else if (memcmp(config, "radio.fem.rxgain ", 17) == 0) {
+    if (!_board->canControlLoRaFemLna()) {
+      strcpy(reply, "Error: unsupported");
+    } else if (memcmp(&config[17], "on", 2) == 0) {
+      if (_board->setLoRaFemLnaEnabled(true)) {
+        _prefs->radio_fem_rxgain = 1;
+        savePrefs();
+        strcpy(reply, "OK - LoRa FEM RX gain on");
+      } else {
+        strcpy(reply, "Error: failed to apply LoRa FEM RX gain");
+      }
+    } else if (memcmp(&config[17], "off", 3) == 0) {
+      if (_board->setLoRaFemLnaEnabled(false)) {
+        _prefs->radio_fem_rxgain = 0;
+        savePrefs();
+        strcpy(reply, "OK - LoRa FEM RX gain off");
+      } else {
+        strcpy(reply, "Error: failed to apply LoRa FEM RX gain");
+      }
+    } else {
+      strcpy(reply, "Error: state must be on or off");
+    }
   } else if (memcmp(config, "agc.reset.interval ", 19) == 0) {
     _prefs->agc_reset_interval = atoi(&config[19]) / 4;
     savePrefs();
@@ -1272,6 +1294,12 @@ void CommonCLI::handleGetCmd(uint32_t sender_timestamp, char* command, char* rep
     sprintf(reply, "> %d", (uint32_t) _prefs->interference_threshold);
   } else if (memcmp(config, "cad", 3) == 0) {
     sprintf(reply, "> %s", _prefs->cad_enabled ? "on" : "off");
+  } else if (memcmp(config, "radio.fem.rxgain", 16) == 0) {
+    if (!_board->canControlLoRaFemLna()) {
+      strcpy(reply, "Error: unsupported");
+    } else {
+      sprintf(reply, "> %s", _board->isLoRaFemLnaEnabled() ? "on" : "off");
+    }
   } else if (memcmp(config, "agc.reset.interval", 18) == 0) {
     sprintf(reply, "> %d", ((uint32_t) _prefs->agc_reset_interval) * 4);
   } else if (memcmp(config, "multi.acks", 10) == 0) {

variants/heltec_v4/HeltecV4Board.cpp

@@ -83,3 +83,20 @@ void HeltecV4Board::begin() {
     return loRaFEMControl.getFEMType() == KCT8103L_PA ? "Heltec V4.3 OLED" : "Heltec V4 OLED";
 #endif
   }
+
+  bool HeltecV4Board::setLoRaFemLnaEnabled(bool enable) {
+    if (!loRaFEMControl.isLnaCanControl()) {
+      return false;
+    }
+    loRaFEMControl.setLNAEnable(enable);
+    loRaFEMControl.setRxModeEnable();
+    return true;
+  }
+
+  bool HeltecV4Board::canControlLoRaFemLna() const {
+    return loRaFEMControl.isLnaCanControl();
+  }
+
+  bool HeltecV4Board::isLoRaFemLnaEnabled() const {
+    return loRaFEMControl.isLNAEnabled();
+  }

variants/heltec_v4/HeltecV4Board.h

@@ -36,4 +36,8 @@ public:
   }
   float getAdcMultiplier() const override { return adc_mult; }
   const char* getManufacturerName() const override;
+
+  bool setLoRaFemLnaEnabled(bool enable) override;
+  bool canControlLoRaFemLna() const override;
+  bool isLoRaFemLnaEnabled() const override;
 };

variants/heltec_v4/LoRaFEMControl.h

@@ -18,7 +18,8 @@ class LoRaFEMControl
     void setRxModeEnable(void);
     void setRxModeEnableWhenMCUSleep(void);
     void setLNAEnable(bool enabled);
-    bool isLnaCanControl(void) { return lna_can_control; }
+    bool isLnaCanControl(void) const { return lna_can_control; }
+    bool isLNAEnabled(void) const { return lna_enabled; }
     void setLnaCanControl(bool can_control) { lna_can_control = can_control; }
     LoRaFEMType getFEMType(void) const { return fem_type; }
   private: