camera: previewRotationQuarterTurns + async previewSize event

Black-screen + extra-90°-rotation on Android both came from
AVFoundation vs CameraX behaving differently at the preview output:

  - AVFoundation: data-output connection's `videoOrientation`
    pre-rotates sample buffers. The Flutter Texture displays them
    upright; `device.activeFormat` reports the sensor-native size
    synchronously.
  - CameraX: the SurfaceProvider hands back a Surface; CameraX
    writes raw sensor frames into it. Rotation is a *transform hint*
    via Preview.setTargetRotation that consumers must apply
    themselves. And the final negotiated resolution isn't known
    until the first SurfaceRequest fires — which happens AFTER
    bindToLifecycle, AFTER lifecycle.start, async on the camera
    executor. So `create` was returning Size(0,0).

Surface extension to bridge the gap:

  - UxCameraValue.previewRotationQuarterTurns (int 0/1/2/3).
    iOS native always emits 0; Android native emits
    `(sensorRotationDegrees / 90) % 4` for the active camera.
    [UxCameraPreview] wraps the Texture in a RotatedBox by that many
    quarter-turns (applied *before* the front-cam mirror so the
    flip lives in screen space, not sensor space).

  - UxCameraPreviewSizeChanged event. Android emits this from
    PreviewSink.onResize whenever a SurfaceRequest carries a new
    resolution; the controller copies it into value.previewSize.
    First emission is what unblocks the camera_thumb's SizedBox
    from its initial 0x0 = "render nothing" state.

  - UxCameraBackend.setDescription's return changed from `Size` to
    `({Size previewSize, int previewRotationQuarterTurns})` so
    a lens swap can both update the rotation and signal that a new
    previewSizeChanged event is incoming.

iOS continues to send previewSize in the create result (the active
format is known synchronously); no previewSizeChanged emission is
needed there. The new field is set to 0 in both create and
setDescription results on iOS.

test/camera/camera_channel_test.dart

@@ -81,16 +81,18 @@ void main() {
       expect(calls.every((c) => (c.arguments as Map)['handle'] == 11), isTrue);
     });
 
-    test('setDescription returns the new previewSize', () async {
+    test('setDescription returns previewSize + rotation', () async {
       handle((_) => {
             'previewSize': {'width': 1280, 'height': 720},
+            'previewRotationQuarterTurns': 1,
           });
 
-      final size = await backend.setDescription(7, 'next');
+      final r = await backend.setDescription(7, 'next');
 
       expect(calls.single.method, 'setDescription');
       expect(calls.single.arguments, {'handle': 7, 'cameraId': 'next'});
-      expect(size, const Size(1280, 720));
+      expect(r.previewSize, const Size(1280, 720));
+      expect(r.previewRotationQuarterTurns, 1);
     });
 
     test('setFlashMode encodes the enum', () async {