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.

.gitignore

@@ -6,4 +6,9 @@
 .pub/
 
 build/
-.cxx/
+.cxx/
+
+# Mirror destinations populated by `prepare_command` in the iOS/macOS
+# podspecs from `darwin/Camera/` — see those podspecs for context.
+ios/Classes/Camera-shared/
+macos/Classes/Camera-shared/