camera: iOS implementation (Phase 2+3)

Native plugin owning AVCaptureSession + AVAssetWriter, mirroring
telegram-iOS's Camera module decomposition. Photo + video capture with
the writer-track transform set from a per-call orientation snapshot
(the three-way preview/capture/device split that camera_avfoundation
can't give us).

Modules:
  CameraPlugin           channels + per-handle instance map
  CameraInstance         session + texture + outputs + recorder
  CameraSession          AVCaptureSession + runtime-error/interrupt obs
  CaptureDevice          front/back discovery, per-device config
  PhotoOutput            AVCapturePhotoOutput, per-shot orientation
  VideoRecorder          AVAssetWriter, lazy inputs, pending-audio queue,
                         stop()/cancel() pair (matches telegram)
  PreviewSink            CVPixelBuffer → FlutterTexture
  AudioSession           setCategory + setActive(true) (only-widen)
  DeviceOrientationBridge

Recorder details:
  - Lazy videoInput/audioInput on first sample, sourceFormatHint:.
  - Audio settings derived from CMAudioFormatDescriptionGet*
    + recommendedAudioSettingsForAssetWriter, gated startWriting.
  - Stop sets stopSampleTime; next sample crossing it triggers
    maybeFinish → finishWriting. No watchdog — telegram pattern.
  - cancel() drops pending audio + cancelWriting + deletes file,
    used by CameraInstance.dispose when teardown finds in-flight
    recording.
  - Diagnostic stream → ux/camera/events {event: "diagnostic"}.

Dart surface extensions over Phase 1:
  - UxCameraValue.audioPermissionGranted
  - UxCameraController.refreshAudioPermission()
  - Static UxCameraController.audioPermissionGranted() /
    openSystemSettings()
  - UxCameraDiagnostic event variant
  - FakeUxCameraBackend.{emitDiagnostic, audioPermission,
    openSettingsCalls}

Tests: 32/32 in test/camera (controller + channel) green.

ios/ux.podspec

@@ -1,13 +1,13 @@
 Pod::Spec.new do |s|
   s.name             = 'ux'
   s.version          = '0.9.0'
-  s.summary          = 'UX Kit — Flutter plugin: keyboard, sensor, file, and QR scanner.'
+  s.summary          = 'UX Kit — Flutter plugin: keyboard, sensor, file, QR scanner, and camera.'
   s.homepage         = 'https://swipelab.co/ux.html'
   s.license          = { :file => '../LICENSE' }
   s.author           = { 'Swipelab' => 'hello@swipelab.co' }
   s.source           = { :path => '.' }
   s.source_files     = 'Classes/**/*.swift'
-  s.frameworks       = ['Photos', 'PhotosUI']
+  s.frameworks       = ['AVFoundation', 'CoreMedia', 'CoreVideo', 'Photos', 'PhotosUI']
   s.dependency 'Flutter'
   s.ios.deployment_target = '13.0'
   s.swift_version = '5.0'