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`).

macos/Classes/Camera/AVCaptureConnection+macOS.swift

@@ -0,0 +1,17 @@
+import AVFoundation
+
+/// macOS counterpart of `AVCaptureConnection+iOS.swift`.
+///
+/// macOS desktop cameras are physically fixed landscape — applying any
+/// `AVCaptureVideoOrientation` would skew the captured photo (and on
+/// some macOS versions the preview's data-output buffers) by 90°.
+/// The orientation snapshot from Flutter (always `portraitUp` on
+/// macOS — desktops don't rotate) is therefore ignored at the
+/// connection layer; the recorded video's track transform is still
+/// `.identity` from VideoRecorder's existing mapping, so video stays
+/// landscape too.
+extension AVCaptureConnection {
+    func applyUxCaptureOrientation(_ orientation: DeviceOrientationFlutter) {
+        // Intentionally empty.
+    }
+}