refactor(mcu): PR-W F-6.7 — privatize setADTR1107Mode

API hygiene. setADTR1107Mode flips ADTR1107 PA/LNA bias registers but
does NOT touch the per-channel ADAR1000 RX/TX enable bits. Production
always reaches it through setAllDevicesTXMode / setAllDevicesRXMode,
which emit both halves. Leaving setADTR1107Mode public after F-6.1
removed the other public mode-switch wrappers invited a future caller
to invoke it directly and end up in a mismatched bias-vs-enable state.

Move the declaration to the private section with a short comment
explaining why the wrappers are the only sanctioned entry point.

9_Firmware/9_1_Microcontroller/9_1_1_C_Cpp_Libraries/ADAR1000_Manager.h

@@ -28,7 +28,6 @@ public:
     // Device Control
     bool setAllDevicesTXMode();
     bool setAllDevicesRXMode();
-    void setADTR1107Mode(BeamDirection direction);
 
     // Monitoring and Diagnostics
     float readTemperature(uint8_t deviceIndex);
@@ -104,7 +103,10 @@ public:
     uint8_t adarAdcRead(uint8_t deviceIndex, uint8_t broadcast);
 
 private:
-
+    // setADTR1107Mode is the wrapper-internal mode switch. Public callers must
+    // go through setAllDevicesTXMode() / setAllDevicesRXMode() so the per-channel
+    // ADAR1000 RX/TX enables stay in sync with the ADTR1107 bias state.
+    void setADTR1107Mode(BeamDirection direction);
 };
 
 // Register Definitions