android: fix inverted device-orientation mapping (real accelerometer reads world-up; emulator uses gravity convention)

android/app/src/main/kotlin/com/mag160c/thermal/ui/DeviceOrientation.kt

@@ -13,8 +13,15 @@ import kotlinx.coroutines.flow.StateFlow
  * Physical orientation of the phone relative to its portrait grip, from the
  * accelerometer.  [deg] is the CLOCKWISE rotation of the device as seen by
  * the user: 0 = upright portrait, 90 = turned clockwise (portrait top edge
- * points to the user's right, gravity along +X device), 180 = upside down,
- * 270 = turned counter-clockwise.
+ * points to the user's right, sensor reads -g on X), 180 = upside down
+ * (sensor reads -g on Y), 270 = turned counter-clockwise (sensor reads +g
+ * on X).
+ *
+ * Android accelerometer convention (REAL devices): at rest the reading is
+ * "acceleration minus gravity", i.e. it points to world UP in device coords
+ * — flat on a table screen-up -> z=+9.81, upright portrait -> y=+9.81.
+ * (Note the emulator's virtual sensor uses the opposite, gravity-vector
+ * convention; do not "fix" this mapping to match emulator defaults.)
  *
  * The activity is portrait-locked (composition glued to the phone frame, so
  * the thermal image region always matches the lens direction).  UI layers use
@@ -45,11 +52,12 @@ object DeviceOrientation : SensorEventListener {
     override fun onSensorChanged(event: SensorEvent) {
         val gx = event.values[0]
         val gy = event.values[1]
+        // world-up in device coords: (0,+g)=0  (-g,0)=90  (0,-g)=180  (+g,0)=270.
         // hysteresis: only switch pose when the dominant axis clearly wins,
         // so ~45 deg in-between holds keep the previous reading
         val next = when {
-            abs(gx) > abs(gy) + 2.5f -> if (gx > 0) 90 else 270
-            abs(gy) > abs(gx) + 2.5f -> if (gy < 0) 0 else 180
+            abs(gx) > abs(gy) + 2.5f -> if (gx < 0) 90 else 270
+            abs(gy) > abs(gx) + 2.5f -> if (gy > 0) 0 else 180
             else -> _deg.value
         }
         if (next != _deg.value) _deg.value = next