fix(GWC-28): stamp gateway envelope sequence at write, not construction

CreateEnvelope stamped Sequence with an interlocked increment when the
envelope was built, so two concurrent InvokeAsync callers could take 1
and 2 and then enqueue in the order 2, 1 — non-monotonic on the wire,
breaking gateway.md's "monotonic per sender" contract. Benign today
(neither side validates inbound sequence, old GWC-10 still open) but it
would fault healthy sessions the moment worker-side validation lands.

WriteLoopAsync now stamps immediately before _writer.WriteAsync. It is
the outbound channel's single consumer (SingleReader = true), so wire
order and stamp order are the same thing by construction and
_nextSequence drops to a plain ulong with no interlocking. This mirrors
the worker's WRK-04 fix, which the gateway half never received.

Also adds TST-28: a [Theory] pinning that GatewayHello.MaxFrameBytes
carries the configured worker-frame maximum (default + 2 MiB override).
The adoption half is asserted only in the Windows-only worker suite, so
a regression to sending 0 — "older gateway, use default" to the worker —
would silently downgrade the negotiated IPC-02 limit with every CI test
still green. Mutation-checked (hard-coded 0 fails both cases).

Tests: WorkerClientTests.ConcurrentInvokesEmitStrictlyIncreasingSequences
OnTheWire (32 parallel invokes; failed 3/3 pre-fix) and
.StartAsync_SendsGatewayHelloWithConfiguredMaxFrameBytes.

src/ZB.MOM.WW.MxGateway.Server/Workers/WorkerClient.cs

@@ -40,7 +40,9 @@ public sealed class WorkerClient : IWorkerClient
     private readonly ConcurrentDictionary<string, PendingCommand> _pendingCommands = new(StringComparer.Ordinal);
     private readonly SemaphoreSlim _pendingCommandSlots;
     private readonly CancellationTokenSource _stopCts = new();
-    private long _nextSequence;
+    // Touched only by WriteLoopAsync — the single consumer of _outboundEnvelopes — so it needs no
+    // interlocking. See WriteLoopAsync for why the stamp happens there rather than at construction.
+    private ulong _nextSequence;
     private WorkerClientState _state;
     private DateTimeOffset _lastHeartbeatAt;
     private int? _processId;
@@ -404,6 +406,13 @@ public sealed class WorkerClient : IWorkerClient
         {
             await foreach (WorkerEnvelope envelope in _outboundEnvelopes.Reader.ReadAllAsync(_stopCts.Token).ConfigureAwait(false))
             {
+                // GWC-28: stamp the sequence at the point of writing, not when the envelope is built.
+                // Stamping at construction let two concurrent InvokeAsync callers take 1 and 2 and then
+                // enqueue in the order 2, 1 — non-monotonic on the wire, breaking gateway.md's
+                // "monotonic per sender" contract. This loop is the channel's single consumer
+                // (SingleReader = true), so wire order and stamp order are the same thing here and
+                // _nextSequence needs no interlocking. Mirrors the worker's WRK-04 fix.
+                envelope.Sequence = unchecked(++_nextSequence);
                 await _writer.WriteAsync(envelope, _stopCts.Token).ConfigureAwait(false);
             }
         }
@@ -1072,11 +1081,13 @@ public sealed class WorkerClient : IWorkerClient
         string correlationId,
         Action<WorkerEnvelope> setBody)
     {
+        // Sequence is deliberately left unset here: WriteLoopAsync stamps it immediately before the
+        // frame goes out, so the numbers are monotonic in wire order however the callers interleave
+        // between construction and enqueue (GWC-28, mirroring the worker's WRK-04 fix).
         WorkerEnvelope envelope = new()
         {
             ProtocolVersion = _connection.FrameOptions.ProtocolVersion,
             SessionId = SessionId,
-            Sequence = (ulong)Interlocked.Increment(ref _nextSequence),
             CorrelationId = correlationId,
         };
         setBody(envelope);

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Workers/WorkerClientTests.cs

@@ -4,6 +4,7 @@ using ZB.MOM.WW.MxGateway.Contracts;
 using ZB.MOM.WW.MxGateway.Contracts.Proto;
 using ZB.MOM.WW.MxGateway.Server.Metrics;
 using ZB.MOM.WW.MxGateway.Server.Workers;
+using ZB.MOM.WW.MxGateway.Tests.Gateway.Workers.Fakes;
 using ZB.MOM.WW.MxGateway.Tests.TestSupport;
 
 namespace ZB.MOM.WW.MxGateway.Tests.Gateway.Workers;
@@ -29,6 +30,33 @@ public sealed class WorkerClientTests
         Assert.Equal(WorkerProcessId, client.ProcessId);
     }
 
+    /// <summary>
+    ///     The <c>GatewayHello</c> carries the negotiated worker-frame maximum so the worker adopts the
+    ///     configured limit instead of its own default (IPC-02); a regression to sending <c>0</c> means
+    ///     "older gateway, use default" to the worker and would silently downgrade the negotiated limit.
+    ///     The adoption half is asserted only in the Windows-only worker suite, so the gateway half is
+    ///     pinned here, in the portable suite (TST-28). Both the default and an override are covered so
+    ///     the assertion tracks configuration rather than a constant.
+    /// </summary>
+    /// <param name="maxMessageBytes">Configured worker-frame maximum to negotiate.</param>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Theory]
+    [InlineData(WorkerFrameProtocolOptions.DefaultMaxMessageBytes)]
+    [InlineData(2 * 1024 * 1024)]
+    public async Task StartAsync_SendsGatewayHelloWithConfiguredMaxFrameBytes(int maxMessageBytes)
+    {
+        await using FakeWorkerHarness harness =
+            await FakeWorkerHarness.CreateConnectedPairAsync(maxMessageBytes: maxMessageBytes);
+        await using WorkerClient client = harness.CreateClient();
+
+        Task startTask = client.StartAsync(CancellationToken.None);
+        WorkerEnvelope gatewayHello = await harness.CompleteStartupAsync().WaitAsync(TestTimeout);
+        await startTask.WaitAsync(TestTimeout);
+
+        Assert.NotEqual(0u, gatewayHello.GatewayHello.MaxFrameBytes);
+        Assert.Equal((uint)maxMessageBytes, gatewayHello.GatewayHello.MaxFrameBytes);
+    }
+
     /// <summary>Verifies that InvokeAsync completes a pending command when a matching reply arrives.</summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]
@@ -141,6 +169,48 @@ public sealed class WorkerClientTests
         Assert.Equal(MxCommandKind.GetWorkerInfo, reply.Reply.Kind);
     }
 
+    /// <summary>
+    ///     The envelope <c>sequence</c> is a monotonic per-sender counter (gateway.md), so the values
+    ///     observed on the pipe must be strictly increasing in wire order. Stamping the sequence when
+    ///     the envelope is constructed lets two concurrent invokes stamp 1 and 2 but enqueue 2 then 1
+    ///     (GWC-28); stamping on the single-consumer write loop makes wire order and sequence order the
+    ///     same thing by construction.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task ConcurrentInvokesEmitStrictlyIncreasingSequencesOnTheWire()
+    {
+        const int commandCount = 32;
+        await using PipePair pipePair = await PipePair.CreateAsync();
+        await using WorkerClient client = CreateClient(pipePair);
+        await CompleteHandshakeAsync(client, pipePair);
+
+        Task<WorkerCommandReply>[] invokeTasks = Enumerable.Range(0, commandCount)
+            .Select(_ => Task.Run(async () => await client.InvokeAsync(
+                CreateCommand(MxCommandKind.Ping),
+                TestTimeout,
+                CancellationToken.None)))
+            .ToArray();
+
+        ulong previousSequence = 0;
+        for (int index = 0; index < commandCount; index++)
+        {
+            WorkerEnvelope commandEnvelope = await pipePair.WorkerReader.ReadAsync().AsTask().WaitAsync(TestTimeout);
+            Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerCommand, commandEnvelope.BodyCase);
+            Assert.True(
+                commandEnvelope.Sequence > previousSequence,
+                $"Command {index} arrived with sequence {commandEnvelope.Sequence} after {previousSequence}; "
+                + "envelope sequences must be strictly increasing in wire order.");
+            previousSequence = commandEnvelope.Sequence;
+
+            await pipePair.WorkerWriter.WriteAsync(
+                CreateCommandReplyEnvelope(commandEnvelope.CorrelationId, MxCommandKind.Ping));
+        }
+
+        await Task.WhenAll(invokeTasks).WaitAsync(TestTimeout);
+        Assert.Equal(WorkerClientState.Ready, client.State);
+    }
+
     /// <summary>Verifies that ReadEventsAsync yields events in pipe order from the worker.</summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]