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.

lib/src/camera/camera_backend.dart

@@ -47,8 +47,13 @@ abstract class UxCameraBackend {
   Future<void> disposeInstance(int handle);
 
   /// Swap to a different camera mid-session. Resets the lock and clears
-  /// any pending recording. Returns the new preview size.
-  Future<Size> setDescription(int handle, String cameraId);
+  /// any pending recording. Returns the new preview size and rotation;
+  /// the size may again be `Size.zero` initially on Android, with a
+  /// follow-up [UxCameraPreviewSizeChanged] event.
+  Future<({Size previewSize, int previewRotationQuarterTurns})> setDescription(
+    int handle,
+    String cameraId,
+  );
 
   /// Set the flash mode used for the next [takePicture]. On front cameras
   /// without a screen-flash fallback the backend silently no-ops; the
@@ -104,11 +109,21 @@ class UxCameraCreateResult {
     required this.handle,
     required this.textureId,
     required this.previewSize,
+    this.previewRotationQuarterTurns = 0,
   });
 
   final int handle;
   final int textureId;
+
+  /// Initial preview size; may be `Size.zero` when the native side
+  /// can't determine it synchronously (Android CameraX needs the
+  /// first `SurfaceRequest` to fire before it knows). In that case
+  /// a [UxCameraPreviewSizeChanged] event follows.
   final Size previewSize;
+
+  /// Number of 90° CW rotations the Texture widget needs. iOS: 0.
+  /// Android: derived from the selected camera's sensor orientation.
+  final int previewRotationQuarterTurns;
 }
 
 /// Events pushed by the native side over `ux/camera/events`. Sealed —
@@ -138,6 +153,16 @@ class UxCameraSessionResumed extends UxCameraEvent {
   const UxCameraSessionResumed(super.handle);
 }
 
+/// Fired when the native side learns or revises the preview's pixel
+/// dimensions. Android emits this after CameraX's first
+/// `SurfaceRequest` resolves (the size isn't known at `create` time);
+/// iOS doesn't emit it (the size lands in the `create` result
+/// synchronously from `device.activeFormat`).
+class UxCameraPreviewSizeChanged extends UxCameraEvent {
+  const UxCameraPreviewSizeChanged(super.handle, this.previewSize);
+  final Size previewSize;
+}
+
 /// Free-text diagnostic message from the native recorder. Routed by
 /// the controller to `Log.tag('camera').i(...)` so it lands in the
 /// log_server pipeline (`~/banlu/tools/log_server/data/banlu.jsonl`).