camera: macOS port via darwin/ split (no shared-file pragmas)

Reuse the AVFoundation Swift files between iOS and macOS without
sprinkling `#if canImport(UIKit)` through them. The split is:

  darwin/Camera/                 platform-shared (AVFoundation only)
    CameraPlugin                 channel + instance map
    CameraInstance               session + outputs + texture
    CameraSession                AVCaptureSession + runtime-error obs
    CaptureDevice                front/back discovery
    PhotoOutput                  AVCapturePhotoOutput
    PreviewSink                  CVPixelBuffer → FlutterTexture
    VideoRecorder                AVAssetWriter
    DeviceOrientation            wire-string enum

  ios/Classes/Camera/            iOS-only impls + extensions
    AudioSession                 AVAudioSession.upgradeForRecording
    DeviceOrientationBridge      UIDevice.orientation listener
    CameraSession+iOS            AVCaptureSessionWasInterrupted obs
                                 + InterruptionReason decode + the
                                 application-audio-session flags
                                 (all iOS-only on AVCaptureSession)
    CameraSettings               UIApplication.openSettingsURLString
    FlutterRegistrar+iOS         method-form of textures/messenger

  macos/Classes/Camera/          macOS no-op stubs (same surface)
    AudioSession                 no-op (no AVAudioSession on macOS)
    DeviceOrientationBridge      no-op (desktops don't rotate)
    CameraSession+macOS          no-op setupPlatform()
    CameraSettings               NSWorkspace → System Settings'
                                 Privacy_Camera pane
    FlutterRegistrar+macOS       property-form of textures/messenger

`CameraSession.init` now calls `setupPlatform()` which each platform
provides via an extension — keeps the iOS-only interruption observer
and the `automaticallyConfiguresApplicationAudioSession` /
`usesApplicationAudioSession` flags (both iOS-only on AVCaptureSession)
out of the shared file. Flash-mode in PhotoOutput uses
`if #available(macOS 11/13, *)` rather than `#if`, since those are
plain version gates not platform splits.

The shared files compile into the iOS pod from `ios/Classes/Camera-shared/`
and into the macOS pod from `macos/Classes/Camera-shared/`, each a
mirror populated by a `prepare_command` in the podspec:

    rm -rf Classes/Camera-shared && cp -R ../darwin/Camera Classes/Camera-shared

Symlinks and `../` source globs both fail — Pathname.glob bails on
symlinks, and CocoaPods silently drops paths that escape the pod
directory. The mirror destinations are .gitignore'd.

macOS UxPlugin now registers CameraPlugin alongside the others.

ios/Classes/Camera/DeviceOrientationBridge.swift

@@ -1,18 +1,19 @@
 import AVFoundation
 import UIKit
 
-/// Translates between Flutter's `DeviceOrientation` (4 enum values
-/// shipped as strings across the channel) and AVFoundation's
-/// `AVCaptureVideoOrientation`, and bridges physical-device rotation
-/// notifications to a closure callback.
-///
-/// Observed orientation source is `UIDevice.current.orientation` —
-/// independent of any UI orientation lock, so this fires even while
-/// the app's window is portrait-locked. `.faceUp` / `.faceDown` are
-/// ignored (no useful direction).
+/// Bridges physical-device rotation notifications to a closure callback.
+/// Observed source is `UIDevice.current.orientation` — independent of
+/// any UI orientation lock, so this fires even while the app's window
+/// is portrait-locked. `.faceUp` / `.faceDown` are ignored (no useful
+/// direction).
 ///
 /// `UIDevice.beginGeneratingDeviceOrientationNotifications()` must be
 /// called on main, balanced with `end…()`; this class enforces both.
+///
+/// iOS-only. The macOS counterpart in
+/// `macos/Classes/Camera/DeviceOrientationBridge.swift` is a no-op —
+/// desktops don't rotate, so `current` stays at the initial
+/// `portraitUp` and the listener never fires.
 final class DeviceOrientationBridge {
     typealias Listener = (DeviceOrientationFlutter) -> Void
 
@@ -74,23 +75,7 @@ final class DeviceOrientationBridge {
     deinit { stop() }
 }
 
-/// Mirrors Flutter's `DeviceOrientation` — the four cardinal values
-/// that travel over the `ux/camera` channel as wire strings. `public`
-/// so the host app's XCTest target can verify the
-/// `DeviceOrientationFlutter` / `AVCaptureVideoOrientation` mapping
-/// without `@testable import`.
-public enum DeviceOrientationFlutter: String {
-    case portraitUp
-    case landscapeLeft
-    case portraitDown
-    case landscapeRight
-
-    /// Parse a wire string. Returns `.portraitUp` for unknown inputs
-    /// (matches the Dart-side fallback in `MethodChannelUxCameraBackend`).
-    public static func parse(_ raw: String?) -> DeviceOrientationFlutter {
-        return DeviceOrientationFlutter(rawValue: raw ?? "") ?? .portraitUp
-    }
-
+extension DeviceOrientationFlutter {
     /// `UIDeviceOrientation` → Flutter convention is a direct 1:1 by
     /// name. Despite the AV-side flip, `UIDeviceOrientation.landscapeLeft`
     /// and Flutter's `landscapeLeft` describe the same physical pose
@@ -105,16 +90,4 @@ public enum DeviceOrientationFlutter: String {
         default: return nil
         }
     }
-
-    /// AVFoundation video orientation. Translates Flutter's portrait-
-    /// relative convention to AVFoundation's hardware-relative one.
-    /// Used to drive `AVCaptureConnection.videoOrientation`.
-    public var avVideoOrientation: AVCaptureVideoOrientation {
-        switch self {
-        case .portraitUp: return .portrait
-        case .portraitDown: return .portraitUpsideDown
-        case .landscapeLeft: return .landscapeRight
-        case .landscapeRight: return .landscapeLeft
-        }
-    }
 }