camera: per-platform capture-orientation extension + macOS sensor=0

macOS preview was stretching (aspect wrong) and macOS photo capture
was rotating the landscape sensor 90° because the shared
PhotoOutput / CameraInstance code was setting
`AVCaptureConnection.videoOrientation` from the orientation snapshot
unconditionally. iOS needs that to rotate sample buffers to portrait;
macOS desktop cams are physically landscape and any rotation just
skews the result.

Moved the rotation call behind a per-platform extension on
`AVCaptureConnection`:
  - `ios/Classes/Camera/AVCaptureConnection+iOS.swift` applies the
    snapshot orientation (current behavior).
  - `macos/Classes/Camera/AVCaptureConnection+macOS.swift` is a
    no-op. macOS-flavoured photos / preview frames now flow at
    native landscape orientation.

`CaptureDevice` reports sensorOrientation=0 on macOS (was hardcoded
90 for iOS); on macOS the page's `normalizeCameraCapture` math then
collapses to identity and the saved JPEG stays the landscape the
sensor produced. iOS keeps sensorOrientation=90 (matches
camera_avfoundation's reported value and the existing capture-
transform math).

Photo and video paths now both produce upright content on macOS
(video already worked because VideoRecorder's transform table maps
the always-portraitUp macOS snapshot to `.identity`).

darwin/Camera/CaptureDevice.swift

@@ -28,18 +28,22 @@ enum CaptureDevice {
                 DiscoveredCamera(
                     device: device,
                     lens: lensName(for: device.position),
-                    // iOS doesn't expose sensor orientation directly;
-                    // 90° matches what `camera_avfoundation` reports
-                    // and what banlu's `normalizeCameraCapture` math
-                    // assumes for iOS sensors. macOS desktop cameras
-                    // are already landscape — 0 is the right answer
-                    // there but the value is unused on macOS (no
-                    // recording rotation, no preview rotation).
-                    sensorOrientation: 90
+                    sensorOrientation: defaultSensorOrientation
                 )
             }
     }
 
+    /// Hardcoded per platform: 90° on iOS (matches the value
+    /// `camera_avfoundation` reports and that banlu's
+    /// `normalizeCameraCapture` math assumes), 0° on macOS (desktop
+    /// cams are already physically landscape — any non-zero value
+    /// would make `normalizeCameraCapture` rotate the saved JPEG).
+    #if os(iOS)
+    private static let defaultSensorOrientation = 90
+    #else
+    private static let defaultSensorOrientation = 0
+    #endif
+
     /// Per-platform set of device types the discovery session asks
     /// for. iOS only has the built-in cameras; macOS additionally
     /// surfaces externals + Continuity Camera (iPhone-as-webcam).