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/Classes/Camera/AudioSession.swift

@@ -0,0 +1,71 @@
+import AVFoundation
+
+/// Idempotent helpers for upgrading the app's shared `AVAudioSession`
+/// to the union of categories needed for capture without trampling
+/// what other modules (audio player, video_player) had set. Pattern
+/// mirrors `camera_avfoundation`'s `upgradeAudioSessionCategory` —
+/// only ever WIDENS the category, never narrows.
+///
+/// We pair this with the session's
+/// `automaticallyConfiguresApplicationAudioSession = false` +
+/// `usesApplicationAudioSession = true` (set in [CameraSession.init])
+/// so AVCaptureSession doesn't yank the category back.
+enum AudioSession {
+    /// Upgrade the shared category to include `.playAndRecord` (and
+    /// the given options union'd with whatever's already set). No-op
+    /// when the union equals the current state, so this is cheap to
+    /// call on every recording start.
+    static func upgradeForRecording() {
+        upgrade(
+            requestedCategory: .playAndRecord,
+            options: [.defaultToSpeaker, .allowBluetoothA2DP, .allowAirPlay]
+        )
+    }
+
+    private static func upgrade(
+        requestedCategory: AVAudioSession.Category,
+        options: AVAudioSession.CategoryOptions
+    ) {
+        let playCategories: Set<AVAudioSession.Category> = [.playback, .playAndRecord]
+        let recordCategories: Set<AVAudioSession.Category> = [.record, .playAndRecord]
+        let currentCategory = AVAudioSession.sharedInstance().category
+        let requiredCategories: Set<AVAudioSession.Category> = [
+            requestedCategory, currentCategory,
+        ]
+
+        let requiresPlay = !requiredCategories.isDisjoint(with: playCategories)
+        let requiresRecord = !requiredCategories.isDisjoint(with: recordCategories)
+
+        var finalCategory = requestedCategory
+        if requiresPlay && requiresRecord {
+            finalCategory = .playAndRecord
+        } else if requiresPlay {
+            finalCategory = .playback
+        } else if requiresRecord {
+            finalCategory = .record
+        }
+
+        let finalOptions = AVAudioSession.sharedInstance().categoryOptions
+            .union(options)
+
+        if finalCategory == currentCategory
+            && finalOptions == AVAudioSession.sharedInstance().categoryOptions
+        {
+            return
+        }
+
+        try? AVAudioSession.sharedInstance().setCategory(
+            finalCategory,
+            options: finalOptions
+        )
+        // With AVCaptureSession.usesApplicationAudioSession = true and
+        // automaticallyConfiguresApplicationAudioSession = false, the
+        // app owns activation — without this, the mic input never
+        // delivers sample buffers. Telegram does the same from
+        // ManagedAudioSession.activate (setActive(true)).
+        try? AVAudioSession.sharedInstance().setActive(
+            true,
+            options: [.notifyOthersOnDeactivation]
+        )
+    }
+}