refactor: M2-Call-Pilot – Module call_signaling + voice_channel mit Fassaden (Entscheidung 2)

- CallSignalingService/callSignalingProvider und VoiceChannelService/
  voiceChannelProvider als einzige oeffentliche Schnittstellen
- voip_manager.dart / livekit_call_manager.dart in ihre Module verschoben,
  Logik unangetastet (nur Klassen-Koepfe, @override, Imports, Provider-Namen)
- Alle Konsumenten entkoppelt; Kompositions-Punkt matrix_client.dart als
  dokumentierte Ausnahme. analyze sauber, 29 Tests gruen, Windows-Start ok.
  Call-Klick-/Geraetetest steht aus (PROGRESS.md)

lib/features/call/voice_channel.dart

@@ -6,8 +6,8 @@ import 'package:collection/collection.dart';
 import 'package:pyramid/core/app_state.dart';
 import 'package:pyramid/widgets/screen_share_picker.dart';
 import 'package:pyramid/core/theme.dart';
-import 'package:pyramid/core/livekit_call_manager.dart';
-import 'package:pyramid/core/voip_manager.dart';
+import 'package:pyramid/features/call_signaling/call_signaling_service.dart';
+import 'package:pyramid/features/voice_channel/voice_channel_service.dart';
 import 'package:flutter_webrtc/flutter_webrtc.dart' as rtc;
 import 'package:pyramid/widgets/hover_region.dart';
 import 'package:pyramid/widgets/pyramid_loader.dart';
@@ -45,8 +45,8 @@ class VoiceChannelView extends ConsumerWidget {
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final pt = PyramidTheme.of(context);
-    final call = ref.watch(callStateProvider);
-    final voip = ref.watch(voipStateProvider);
+    final call = ref.watch(voiceChannelProvider);
+    final voip = ref.watch(callSignalingProvider);
 
     if (voip.currentCall != null && voip.currentCall!.state != CallState.kEnded) {
       return _UnifiedCallView(
@@ -101,7 +101,7 @@ class VoiceChannelView extends ConsumerWidget {
     );
   }
 
-  List<_ParticipantData> _getLiveKitParticipants(LiveKitCallManager call) {
+  List<_ParticipantData> _getLiveKitParticipants(VoiceChannelService call) {
     final room = call.room;
     if (room == null) return [];
     final all = <Participant>[
@@ -142,7 +142,7 @@ class VoiceChannelView extends ConsumerWidget {
     }).toList();
   }
 
-  List<_ParticipantData> _getVoipParticipants(PyramidVoipManager voip) {
+  List<_ParticipantData> _getVoipParticipants(CallSignalingService voip) {
     final participants = <_ParticipantData>[];
 
     double ar(rtc.RTCVideoRenderer r) {
@@ -997,7 +997,7 @@ class _FullscreenVideoPage extends StatelessWidget {
   }
 }
 
-void _handleLiveKitScreenShare(BuildContext context, LiveKitCallManager call) async {
+void _handleLiveKitScreenShare(BuildContext context, VoiceChannelService call) async {
   if (call.isScreenSharing) {
     await call.stopScreenShare();
   } else {
@@ -1006,7 +1006,7 @@ void _handleLiveKitScreenShare(BuildContext context, LiveKitCallManager call) as
   }
 }
 
-void _showLiveKitQualityMenu(BuildContext context, LiveKitCallManager call, Offset pos) {
+void _showLiveKitQualityMenu(BuildContext context, VoiceChannelService call, Offset pos) {
   final pt = PyramidTheme.of(context);
   showMenu(
     context: context,
@@ -1025,7 +1025,7 @@ void _showLiveKitQualityMenu(BuildContext context, LiveKitCallManager call, Offs
   );
 }
 
-void _showVoipQualityMenu(BuildContext context, PyramidVoipManager voip, Offset pos) {
+void _showVoipQualityMenu(BuildContext context, CallSignalingService voip, Offset pos) {
   final pt = PyramidTheme.of(context);
   showMenu(
     context: context,