fix(IPC-39): make the CommandTooLarge pre-check allow for the sequence stamp

The pre-check measures the command envelope while Sequence is still 0, and an
unset protobuf field costs nothing to encode -- so the measured size is a lower
bound on the frame WriteLoopAsync emits after GWC-28 stamps the sequence. Being
a lower bound is the right kind of conservative for the knownSize question the
comment answers, and the wrong kind for the pre-check itself: it admits an
envelope in the last eleven bytes of the range that the writer then rejects with
a session-fatal MessageTooLarge.

Compare against MaxMessageBytes - MaxSequenceEncodingBytes (1 tag byte + a
10-byte varint, the widest 64-bit field encoding) and say in the comment that
the pre-check is a lower bound relying on that allowance.

Not reachable under the shipped validator (caller-controlled commands arrive
through gRPC capped at least 64 KiB below the pipe max, gateway-authored
commands are tiny), so this closes the argument rather than a live defect. The
existing CommandTooLarge test still passes unchanged.

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

@@ -237,15 +237,22 @@ public sealed class WorkerClient : IWorkerClient
             // CalculateSize. WriteLoopAsync stamps envelope.Sequence immediately before the write
             // (GWC-28), and a non-zero varint field grows the encoding — so the number computed here
             // is a lower bound on the frame the writer actually emits, never the frame length. Passing
-            // it as a knownSize would under-length the prefix and desync the worker's framing. The
+            // it as a knownSize would under-length the prefix and desync the worker's framing.
-            // pre-check stays a pre-check: it is conservative in the right direction.
+            //
+            // IPC-39: being a lower bound is conservative for the knownSize question but the wrong
+            // direction for this pre-check, which must not admit an envelope the writer will then
+            // reject with a session-fatal MessageTooLarge. Compare against the frame maximum less the
+            // MaxSequenceEncodingBytes the stamp can add, so every envelope this check accepts still
+            // fits once the sequence is written.
             int envelopeSize = commandEnvelope.CalculateSize();
-            if (envelopeSize > _connection.FrameOptions.MaxMessageBytes)
+            int preCheckLimit = _connection.FrameOptions.MaxMessageBytes - MaxSequenceEncodingBytes;
+            if (envelopeSize > preCheckLimit)
             {
                 throw new WorkerClientException(
                     WorkerClientErrorCode.CommandTooLarge,
                     $"Worker command {method} serializes to {envelopeSize} bytes, exceeding the negotiated "
-                    + $"worker-frame maximum of {_connection.FrameOptions.MaxMessageBytes} bytes.");
+                    + $"worker-frame maximum of {_connection.FrameOptions.MaxMessageBytes} bytes less the "
+                    + $"{MaxSequenceEncodingBytes}-byte sequence-stamp allowance.");
             }
 
             await EnqueueAsync(commandEnvelope, cancellationToken).ConfigureAwait(false);
@@ -455,6 +462,18 @@ public sealed class WorkerClient : IWorkerClient
         _stopCts.Dispose();
     }
 
+    /// <summary>
+    ///     Worst-case number of bytes <see cref="WriteLoopAsync"/>'s sequence stamp adds to an
+    ///     envelope that was sized before it: one tag byte plus a ten-byte varint, the maximum
+    ///     encoding of a 64-bit protobuf field. The <c>CommandTooLarge</c> pre-check subtracts this
+    ///     from the negotiated frame maximum (IPC-39) because it measures the envelope while
+    ///     <c>Sequence</c> is still 0 — an unset field costs nothing to encode, so the measured size
+    ///     is a lower bound on the frame the writer emits, and without the allowance an envelope in
+    ///     the last eleven bytes of the range would pass the pre-check and then fault the session on
+    ///     the write.
+    /// </summary>
+    private const int MaxSequenceEncodingBytes = 11;
+
     /// <summary>Manages writing envelopes to the worker pipe.</summary>
     private async Task WriteLoopAsync()
     {