fix(diag): probe-owned inbound depth counter - SingleReader channels have no Count

The first armed ACDREAM_PROBE_NET launch died at exit 4 one second into
the world: the [net-tick] line read _inboundQueue.Reader.Count, but the
queue is built with SingleReader=true and that channel implementation
throws NotSupportedException from Count. The net thread now increments
and the frame thread decrements a probe-owned Interlocked counter
instead; behavior with the probe off is untouched.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.Core.Net/WorldSession.cs

@@ -995,6 +995,8 @@ public sealed class WorldSession : IDisposable
         while (_inboundQueue.Reader.TryRead(
                    out PooledInboundDatagram datagram))
         {
+            if (NetDiagnostics.ProbeNet)
+                Interlocked.Decrement(ref _probeInboundDepth);
             try
             {
                 ProcessDatagram(datagram.Memory);
@@ -1034,6 +1036,10 @@ public sealed class WorldSession : IDisposable
     private int _probeBudgetBreaks;
     private int _probeSendWindow;
     private int _probeAckWindow;
+    // Probe-owned queue depth: the SingleReader channel's Reader.Count
+    // throws NotSupportedException, so the net thread increments on
+    // enqueue and the frame thread decrements on dequeue instead.
+    private int _probeInboundDepth;
 
     /// <summary>
     /// #260 probe: accumulate per-Tick cadence facts and emit one
@@ -1067,7 +1073,7 @@ public sealed class WorldSession : IDisposable
         int acks = Interlocked.Exchange(ref _probeAckWindow, 0);
         Console.WriteLine(
             $"[net-tick] in/s={_probeProcessedWindow / windowSeconds:F0}"
-            + $" q={_inboundQueue.Reader.Count}"
+            + $" q={Volatile.Read(ref _probeInboundDepth)}"
             + $" budget-breaks={_probeBudgetBreaks}"
             + $" maxgap={maxGapMs:F0}ms"
             + $" out/s={sends / windowSeconds:F0}"
@@ -1145,6 +1151,8 @@ public sealed class WorldSession : IDisposable
                             new PooledInboundDatagram(
                                 queuedBuffer,
                                 result.Length));
+                    if (ownershipTransferred && NetDiagnostics.ProbeNet)
+                        Interlocked.Increment(ref _probeInboundDepth);
                 }
                 catch (System.Net.Sockets.SocketException ex)
                 {