Fix iOS background audio pacing

Sleep-paced threads stall for hundreds of milliseconds when iOS coalesces a
backgrounded app's wakeups, and the deadline resets that discarded the deficit
left the capture backlog queued until its ring overflowed: regular dropouts
that worsen the longer the app stays backgrounded. Pace both 20 ms hands-offs
from the AVAudioSourceNode render callback instead (mix via Audio.RunCycle
with deviceClockedAudio, capture handoff from the ring), wake the UDP sender
on a queue signal instead of a 1 ms poll, and let stalled consumers drop their
backlog to the buffer target instead of ratcheting it.

clients/apple/VoiceCat.iOS/AppModel.cs

@@ -99,7 +99,10 @@ internal sealed class AppModel
         explicitDisconnect = false; IsConnecting = true; connectedProfile = profile;
         Status = restoring ? "Reconnecting…" : "Connecting…"; Notify();
         lifetime?.Cancel(); lifetime?.Dispose(); lifetime = new();
-        VoiceCatClient next = new("VoiceCat-iOS", "0.0.1", storage.TofuPath);
+        // The AVAudioSourceNode render callback drives Audio.RunCycle and the 20 ms capture
+        // handoff; a sleep-paced audio worker stalls when iOS coalesces backgrounded wakeups and
+        // the call glitches after several minutes in the background.
+        VoiceCatClient next = new("VoiceCat-iOS", "0.0.1", storage.TofuPath, deviceClockedAudio: true);
         next.ConnectionStateChanged += state =>
         {
             if (state == ClientConnectionState.Disconnected && next.ConnectionFailure is { } failure)