refactor(io): unique_ptr ownership for motion sensors and I2C keyboard (#11458)

* refactor(io): unique_ptr ownership for motion sensors and I2C keyboard

- AccelerometerThread / MagnetometerThread: the owned MotionSensor
  becomes unique_ptr, removing the manual delete/null bookkeeping in
  clean(). Deletion behavior is unchanged (MotionSensor's destructor is
  virtual).

- KbI2cBase: the TCA keyboard was a reference member bound to an
  anonymous heap allocation - ownership was invisible and nothing could
  ever free it. It becomes unique_ptr with an out-of-line destructor
  (the base type is only forward-declared in the header).

- GeoCoord::pointAtDistance returned shared_ptr with no shared
  ownership anywhere (and no callers); return by value instead.

No behavior change.

* fix(io): make TCA8418KeyboardBase destructor public for unique_ptr ownership

* refactor(gps): delete dead pointAtDistance instead of converting it

Per review: zero callers in this repo or device-ui, and the math was
wrong at both ends (rangeMetersToRadians multiplies meters by 1852,
treating meters as nautical miles). Remove it, its now-unused helper,
and the <memory> include the old shared_ptr signature pulled in.

src/motion/MagnetometerThread.h

@@ -10,12 +10,14 @@
 #include "MMC5983MASensor.h"
 #include "MotionSensor.h"
 
+#include <memory>
+
 extern ScanI2C::DeviceAddress magnetometer_found;
 
 class MagnetometerThread : public concurrency::OSThread
 {
   private:
-    MotionSensor *sensor = nullptr;
+    std::unique_ptr<MotionSensor> sensor;
     ScanI2C::FoundDevice device;
     bool isInitialised = false;
 
@@ -69,7 +71,7 @@ class MagnetometerThread : public concurrency::OSThread
         switch (device.type) {
 #if __has_include(<SparkFun_MMC5983MA_Arduino_Library.h>)
         case ScanI2C::DeviceType::MMC5983MA:
-            sensor = new MMC5983MASensor(device);
+            sensor.reset(new MMC5983MASensor(device));
             break;
 #endif
         default:
@@ -106,8 +108,7 @@ class MagnetometerThread : public concurrency::OSThread
     void clean()
     {
         isInitialised = false;
-        delete sensor;
-        sensor = nullptr;
+        sensor.reset();
     }
 };