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.

macos/Classes/Camera/AudioSession.swift

@@ -0,0 +1,9 @@
+import Foundation
+
+/// macOS counterpart of `ios/Classes/Camera/AudioSession.swift`.
+/// `AVAudioSession` is an iOS/tvOS/watchOS concept — macOS apps have
+/// no such global category state. `AVCaptureSession` configures its
+/// own audio routing on macOS, so this is a pure no-op.
+enum AudioSession {
+    static func upgradeForRecording() {}
+}

macos/Classes/Camera/CameraSession+macOS.swift

@@ -0,0 +1,22 @@
+import AVFoundation
+import Foundation
+
+/// macOS counterpart of `CameraSession+iOS.swift`. Intentionally a
+/// no-op:
+///
+/// - `automaticallyConfiguresApplicationAudioSession` /
+///   `usesApplicationAudioSession` are iOS-only properties on
+///   `AVCaptureSession`. macOS doesn't have an app-wide
+///   `AVAudioSession`, so there's nothing to "configure".
+/// - `AVCaptureSessionWasInterrupted` and `InterruptionReason` are
+///   iOS/tvOS/watchOS only — desktop AVCaptureSessions never get
+///   "interrupted by a phone call" or "displaced by another app's
+///   exclusive camera hold" in the way iOS does.
+///
+/// `onInterrupted` and `onResumed` on [CameraSession] are present
+/// for surface parity but stay silent on macOS.
+extension CameraSession {
+    func setupPlatform() {
+        // Intentionally empty.
+    }
+}

macos/Classes/Camera/CameraSettings.swift

@@ -0,0 +1,16 @@
+import AppKit
+
+/// macOS counterpart of the iOS `CameraSettings.openAppSettings`.
+/// There's no per-app deep link on macOS; we go straight to the
+/// Camera privacy pane in System Settings. The URL scheme has been
+/// stable since macOS 10.14.
+enum CameraSettings {
+    @MainActor
+    static func openAppSettings() {
+        if let url = URL(
+            string: "x-apple.systempreferences:com.apple.preference.security?Privacy_Camera"
+        ) {
+            NSWorkspace.shared.open(url)
+        }
+    }
+}

macos/Classes/Camera/DeviceOrientationBridge.swift

@@ -0,0 +1,19 @@
+import Foundation
+
+/// macOS counterpart of `ios/Classes/Camera/DeviceOrientationBridge.swift`.
+/// Desktops don't rotate; the bridge is shaped identically to the iOS
+/// one so [CameraInstance] can call it without `#if` blocks, but
+/// `current` stays at `.portraitUp` and the listener never fires.
+final class DeviceOrientationBridge {
+    typealias Listener = (DeviceOrientationFlutter) -> Void
+
+    private(set) var current: DeviceOrientationFlutter = .portraitUp
+
+    func start(listener: @escaping Listener) {
+        // Intentionally empty — no rotation events on macOS.
+    }
+
+    func stop() {
+        // Intentionally empty — nothing to unwind.
+    }
+}

macos/Classes/Camera/FlutterRegistrar+macOS.swift

@@ -0,0 +1,11 @@
+import FlutterMacOS
+
+/// macOS counterpart of `ios/Classes/Camera/FlutterRegistrar+iOS.swift`.
+/// On macOS, `FlutterPluginRegistrar.textures` and `.messenger` are
+/// properties (no parens) rather than methods. Shared `CameraPlugin`
+/// uses `registrar.uxTextures` and `registrar.uxMessenger` for
+/// surface parity.
+extension FlutterPluginRegistrar {
+    var uxTextures: FlutterTextureRegistry { textures }
+    var uxMessenger: FlutterBinaryMessenger { messenger }
+}

macos/Classes/UxPlugin.swift

@@ -9,6 +9,7 @@ public class UxPlugin: NSObject, FlutterPlugin {
             FilePlugin(),
             ClipboardPlugin(),
             GalleryPlugin(),
+            CameraPlugin(),
         ]
         for plugin in plugins {
             plugin.register(with: registrar)