fix(net): survive transient socket errors in the receive loop (2026-07-24 audit review)

WorldSession.NetReceiveLoop wrapped its entire while loop in a single
try/catch, so ANY non-timeout SocketException permanently killed the
background receive thread: the catch block at the loop's end was empty
(misattributed the error to "socket closed during shutdown"), and the
finally called _inboundQueue.Writer.TryComplete(), which silently and
irrecoverably stopped all inbound processing for the rest of the
session — no log line, no recovery path, and LiveSessionHost.Reconnect
has zero production callers to notice.

The realistic trigger is a well-known Windows UdpClient quirk: an ICMP
"port unreachable" reply to an EARLIER Send (e.g. against a stale ACE
session that already tore down its socket) surfaces as a
WSAECONNRESET SocketException on this socket's NEXT, completely
unrelated Receive call. NetClient.Receive already swallows the
expected SocketError.TimedOut heartbeat case; anything else reaching
WorldSession was a real, transient, per-datagram error being treated
as session-fatal.

Three changes, root-cause not a band-aid:

- NetClient's constructor now disables SIO_UDP_CONNRESET reporting on
  Windows, so a delayed ICMP error can't poison receives at all.
- NetReceiveLoop now catches SocketException PER ITERATION, logs it,
  and continues polling instead of exiting. The existing clean-shutdown
  paths (cancellation, ObjectDisposedException during Dispose) are
  unchanged — only the non-timeout-socket-error case that used to kill
  the loop is now recoverable.
- NetClient.Receive no longer calls the ReceiveTimeout setter (a
  setsockopt syscall) on every single call — only when the requested
  timeout differs from the last-applied value, cached in a new field.
  This was an unrelated but adjacent finding (4x/sec syscall churn at
  the 250ms heartbeat cadence) in the same audit.

No change to outbound wire behavior, ack cadence, heartbeat interval,
or datagram ordering — this is purely receive-loop resilience.

Tests: NetClientTests gained a SIO_UDP_CONNRESET construction smoke
test and two ReceiveTimeout-caching tests. A new
WorldSessionNetReceiveLoopResilienceTests drives the actual private
NetReceiveLoop method (via the existing internal
IWorldSessionTransport seam + reflection) with a scripted transport
that throws a non-timeout SocketException on the first call, proving
the loop survives it and keeps enqueueing subsequent datagrams — fully
deterministic, no real sockets. Full solution suite green: 3204/3206
Core, 3462/3465 App, 552/552 Core.Net (skips pre-existing).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
(cherry picked from commit c72ce028a927e15b8a54a86bbd0661a723dc84ca)

tests/AcDream.Core.Net.Tests/WorldSessionNetReceiveLoopResilienceTests.cs

@@ -0,0 +1,102 @@
+using System.Net;
+using System.Net.Sockets;
+using System.Reflection;
+
+namespace AcDream.Core.Net.Tests;
+
+/// <summary>
+/// 2026-07-24 audit fix: <c>WorldSession.NetReceiveLoop</c> used to wrap its
+/// entire while loop in a single try/catch, so a non-timeout
+/// <see cref="SocketException"/> (classically Windows delivering a delayed
+/// WSAECONNRESET off an earlier Send's ICMP port-unreachable) exited the
+/// loop for good and silently killed all inbound processing for the rest of
+/// the session. These tests drive the actual private <c>NetReceiveLoop</c>
+/// method — via the internal <see cref="IWorldSessionTransport"/> seam and
+/// reflection, since the method has no public entry point — with a scripted
+/// transport that throws a non-timeout SocketException on the first call.
+/// No real sockets are involved, so the tests are deterministic.
+/// </summary>
+public sealed class WorldSessionNetReceiveLoopResilienceTests
+{
+    [Fact]
+    public void NetReceiveLoop_NonTimeoutSocketException_LogsAndKeepsReceiving()
+    {
+        var transport = new ScriptedTransport();
+        var session = new WorldSession(
+            new IPEndPoint(IPAddress.Loopback, 9000),
+            transport);
+
+        MethodInfo loopMethod = typeof(WorldSession).GetMethod(
+            "NetReceiveLoop", BindingFlags.NonPublic | BindingFlags.Instance)!;
+
+        // Deliberately does NOT redirect Console.Error: xUnit runs test
+        // classes in this assembly in parallel by default, and Console.Error
+        // is process-global mutable state, so swapping it out here could
+        // race with another class's concurrently running test and either
+        // swallow its output or restore the wrong writer. The behavioral
+        // assertions below (call count + queue drain) fully cover the fix
+        // without needing to observe the log line's exact text.
+        var thread = new Thread(() => loopMethod.Invoke(session, null))
+        {
+            IsBackground = true,
+            Name = "test.net-receive-loop",
+        };
+        thread.Start();
+
+        // The scripted transport throws SocketException on call 1, returns
+        // one datagram on call 2, then throws ObjectDisposedException on
+        // call 3 to end the loop the same way a real shutdown-time
+        // disposal would. If the pre-fix bug were still present, the loop
+        // would exit after call 1 and this thread would never terminate on
+        // its own via the scripted ObjectDisposedException (call 3 would
+        // never be reached).
+        bool exited = thread.Join(TimeSpan.FromSeconds(5));
+        Assert.True(exited, "NetReceiveLoop did not exit after the scripted ObjectDisposedException — " +
+            "the non-timeout SocketException likely killed the loop before it reached call 3.");
+
+        // Proves the loop survived the first (non-timeout) SocketException
+        // and went on to make a second AND third Receive() call, instead of
+        // exiting after call 1 the way the pre-fix code did.
+        Assert.Equal(3, transport.CallCount);
+
+        // The datagram handed back on call 2 must have made it into the
+        // inbound queue — i.e. the loop's normal write path still runs
+        // after recovering from the error.
+        int processed = session.Tick();
+        Assert.Equal(1, processed);
+    }
+
+    private sealed class ScriptedTransport : IWorldSessionTransport
+    {
+        private int _calls;
+
+        public int CallCount => _calls;
+
+        public void Send(ReadOnlySpan<byte> datagram) { }
+
+        public void Send(IPEndPoint remote, ReadOnlySpan<byte> datagram) { }
+
+        public byte[]? Receive(TimeSpan timeout, out IPEndPoint? from)
+        {
+            int n = Interlocked.Increment(ref _calls);
+            from = null;
+            switch (n)
+            {
+                case 1:
+                    // Simulates a non-timeout SocketException, e.g. Windows'
+                    // WSAECONNRESET off a stale peer's ICMP port-unreachable.
+                    throw new SocketException((int)SocketError.ConnectionReset);
+                case 2:
+                    from = new IPEndPoint(IPAddress.Loopback, 9000);
+                    return new byte[] { 0xAA, 0xBB, 0xCC, 0xDD };
+                default:
+                    // Simulates NetClient having been disposed out from
+                    // under the loop during shutdown — the pre-existing,
+                    // still-preserved silent-exit path.
+                    throw new ObjectDisposedException(nameof(ScriptedTransport));
+            }
+        }
+
+        public void Dispose() { }
+    }
+}