sensor: expose orientationListenable as a ValueListenable

Backed by a Dart-side 100ms poll of the existing FFI getter; the polling
timer starts on first listener and stops when the last listener detaches.

lib/src/sensor.dart

@@ -1,6 +1,8 @@
+import 'dart:async';
 import 'dart:ffi';
 import 'dart:io';
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 
 DynamicLibrary? _initLib() {
@@ -34,4 +36,47 @@ class UxSensor {
     }
     return DeviceOrientation.values[idx];
   }
+
+  /// Lazily-created listenable backing [orientationListenable].
+  static _OrientationNotifier? _orientationNotifier;
+
+  /// Listenable form of [orientation], updated as the device rotates.
+  /// Backed by a Dart-side 100ms poll of the underlying FFI getter.
+  static ValueListenable<DeviceOrientation> get orientationListenable {
+    return _orientationNotifier ??= _OrientationNotifier();
+  }
+}
+
+class _OrientationNotifier extends ChangeNotifier
+    implements ValueListenable<DeviceOrientation> {
+  _OrientationNotifier() : _value = UxSensor.orientation;
+
+  Timer? _timer;
+  DeviceOrientation _value;
+
+  @override
+  DeviceOrientation get value => _value;
+
+  @override
+  void addListener(VoidCallback listener) {
+    super.addListener(listener);
+    _timer ??= Timer.periodic(const Duration(milliseconds: 100), _tick);
+  }
+
+  @override
+  void removeListener(VoidCallback listener) {
+    super.removeListener(listener);
+    if (!hasListeners) {
+      _timer?.cancel();
+      _timer = null;
+    }
+  }
+
+  void _tick(Timer _) {
+    final next = UxSensor.orientation;
+    if (next != _value) {
+      _value = next;
+      notifyListeners();
+    }
+  }
 }