Test 3: per-cycle PlaybackDoneFlag + log timeout in fire-and-forget cleanup

Stale Set() from a delayed cleanup task could short-circuit a future
PLAYBACK state; new flag per cycle avoids the cross-cycle leak. Also
log when the 2 s done.Wait timeout fires so silent PortAudio callback
faults are visible during hardware debugging.

tests/03-full-cycle-cs/Program.cs

@@ -96,7 +96,7 @@ try
     int beepFrames = 0;
     int recordOffset = 0;
     short[] recordBuf = null!;  // assigned on BEEPING→RECORDING transition
-    var playbackDone = new PlaybackDoneFlag();
+    PlaybackDoneFlag? playbackDone = null;
 
     try
     {
@@ -157,6 +157,7 @@ try
                     {
                         try
                         {
+                            playbackDone = new PlaybackDoneFlag();
                             FireAndForgetPlayback(device, recordBuf, playbackDone);
                         }
                         catch (Exception ex)
@@ -177,7 +178,7 @@ try
                 case State.Playback:
                 {
                     // Discard input during playback (gate the detector).
-                    if (playbackDone.Consume())
+                    if (playbackDone!.Consume())
                     {
                         model.Reset();
                         Console.WriteLine("[state] IDLE");
@@ -287,7 +288,8 @@ static void FireAndForgetBeep(int device, short[] beepBuffer)
         // 2 s timeout so a failed callback (which PortAudio silently swallows
         // and would leave 'done' unset) eventually releases the stream + handle
         // instead of leaking them for the process lifetime.
-        done.Wait(TimeSpan.FromSeconds(2));
+        if (!done.Wait(TimeSpan.FromSeconds(2)))
+            Console.Error.WriteLine("[error] beep playback timed out — callback may have faulted");
         Thread.Sleep(200);
         stream.Stop();
         stream.Dispose();
@@ -348,7 +350,8 @@ static void FireAndForgetPlayback(int device, short[] recordBuf, PlaybackDoneFla
     Task.Run(() =>
     {
         // Same 2 s safety timeout as the beep cleanup.
-        done.Wait(TimeSpan.FromSeconds(2));
+        if (!done.Wait(TimeSpan.FromSeconds(2)))
+            Console.Error.WriteLine("[error] recording playback timed out — callback may have faulted");
         Thread.Sleep(200);
         stream.Stop();
         stream.Dispose();