feat(videocall): complete channel wrapper + add integration plan doc

Add 11 missing JCMediaChannel methods to VideocallChannelService:
getSelfParticipant, subscribeParticipantAudio, startScreenShareVideo,
stopScreenShareVideo, enableSelfVideoRatio, getMaxResolution,
sendCommandToDelivery, enableVolumeChangeNotify, getCaptureScreen,
getScreenRenderId, getScreenUserId.

Add VIDEOCALL_INTEGRATION.md with full implementation checklist
tracking phases 1-10.

packages/videocall_sdk/lib/src/services/videocall_channel_service.dart

@@ -298,6 +298,57 @@ class VideocallChannelService with JCMediaChannelCallback {
     return _channel!.getCdnState();
   }
 
+  Future<String> getCaptureScreen() async {
+    if (_channel == null) return '';
+    return _channel!.getCaptureScreen();
+  }
+
+  Future<String> getScreenRenderId() async {
+    if (_channel == null) return '';
+    return _channel!.getScreenRenderId();
+  }
+
+  Future<String> getScreenUserId() async {
+    if (_channel == null) return '';
+    return _channel!.getScreenUserId();
+  }
+
+  // -- Self participant --
+
+  Future<JCMediaChannelParticipant?> getSelfParticipant() async {
+    return _channel?.getSelfParticipant();
+  }
+
+  Future<bool> subscribeParticipantAudio(
+      JCMediaChannelParticipant participant, bool subscribe) async {
+    if (_channel == null) return false;
+    return _channel!.subscribeParticipantAudio(participant, subscribe);
+  }
+
+  // -- Screen share video --
+
+  Future<JCMediaDeviceVideoCanvas?> startScreenShareVideo(
+      int renderType, int pictureSize) async {
+    return _channel?.startScreenShareVideo(renderType, pictureSize);
+  }
+
+  Future<bool> stopScreenShareVideo() async {
+    if (_channel == null) return false;
+    return _channel!.stopScreenShareVideo();
+  }
+
+  // -- Video ratio / resolution --
+
+  Future<bool> enableSelfVideoRatio(bool enable, double ratio) async {
+    if (_channel == null) return false;
+    return _channel!.enableSelfVideoRatio(enable, ratio);
+  }
+
+  Future<int> getMaxResolution() async {
+    if (_channel == null) return 0;
+    return _channel!.getMaxResolution();
+  }
+
   // -- Messaging --
 
   Future<bool> sendMessage(
@@ -311,6 +362,11 @@ class VideocallChannelService with JCMediaChannelCallback {
     return _channel!.sendCommand(name, param);
   }
 
+  Future<bool> sendCommandToDelivery(String command) async {
+    if (_channel == null) return false;
+    return _channel!.sendCommandToDelivery(command);
+  }
+
   // -- Statistics --
 
   Future<String> getStatistics() async {
@@ -318,6 +374,18 @@ class VideocallChannelService with JCMediaChannelCallback {
     return _channel!.getStatistics();
   }
 
+  // -- Volume change notify --
+
+  Future<bool> enableVolumeChangeNotify(bool value) async {
+    if (_channel == null) return false;
+    return _channel!.enableVolumeChangeNotify(value);
+  }
+
+  Future<bool> getVolumeChangeNotify() async {
+    if (_channel == null) return false;
+    return _channel!.getVolumeChangeNotify();
+  }
+
   // -- Dispose --
 
   void dispose() {