fix(WRK-21,WRK-28,WRK-23,IPC-30): byte-budget the DrainEvents reply, stop size errors from killing sessions

WRK-21 — DrainEvents was bounded by event count only, so a byte-heavy queue
(large string/array MxValues) built a reply above the negotiated frame maximum:
the writer rejected the frame, the exception unwound the session, and the events
already dequeued were destroyed. The drain is now byte-budgeted inside the queue
lock, so an event is dequeued only once it is known to fit and one that does not
stays at the head. Truncation is reported through the reply's existing
DiagnosticMessage (no contract change); callers drain until an empty reply. Both
reply-write seams — the control-command path and ProcessCommandAsync — now catch
MessageTooLarge and answer the correlation with an InvalidRequest reply instead
of unwinding or faulting the session. Satisfies IPC-23 R1-R3.

WRK-28 — the 10,000 drain ceiling moves to GatewayContractInfo
.MaxDrainEventsPerCommand, referenced by both the gateway request validator and
the worker clamp, replacing a comment-only sync contract. C# const only; no
.proto change.

WRK-23 — WorkerFrameWriter now peek-stamps, validates, then commits the sequence
counter immediately before the stream write, so a per-frame rejection leaves no
phantom gap on the wire.

IPC-30 — an oversized event frame stays session-fatal (it is undeliverable end to
end and neither dropping nor synthesizing a replacement is allowed), but the
death is structured: the event's identity and sizes are logged (never its value),
a WorkerFault with category PROTOCOL_VIOLATION and command method EventDrain is
written, then the session exits as before.

Docs updated in the same change: MxAccessWorkerInstanceDesign.md (drain byte cap,
truncation contract, oversized-head behavior, oversized-event policy, no control
reply is session-fatal on size), WorkerFrameProtocol.md (reply pre-sizing,
non-fatal reply-size rule, oversized-event policy, rejected frames do not consume
sequence numbers), gateway.md (DrainEvents two-axis bound).

src/ZB.MOM.WW.MxGateway.Worker.Tests/Ipc/WorkerFrameProtocolTests.cs

@@ -410,6 +410,48 @@ public sealed class WorkerFrameProtocolTests
         }
     }
 
+    /// <summary>
+    /// Verifies a per-frame rejection does not burn a sequence number (WRK-23). The sequence is a
+    /// diagnostic counter, so a gap breaks nothing functionally — but an operator correlating a pipe
+    /// capture reads a gap as a lost frame and chases a bug that does not exist, and the gap-free
+    /// guarantee the concurrent-write test asserts would otherwise only hold until the first
+    /// rejection.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task WriteAsync_PerFrameRejection_DoesNotConsumeSequence()
+    {
+        const int maxMessageBytes = 512;
+        WorkerFrameProtocolOptions options = new(
+            SessionId,
+            GatewayContractInfo.WorkerProtocolVersion,
+            Nonce,
+            maxMessageBytes);
+        using MemoryStream stream = new();
+        WorkerFrameWriter writer = new(stream, options);
+
+        await writer.WriteAsync(CreateEventEnvelope());
+
+        WorkerEnvelope oversized = CreateGatewayHelloEnvelope();
+        oversized.GatewayHello.GatewayVersion = new string('x', maxMessageBytes * 2);
+        WorkerFrameProtocolException exception =
+            await Assert.ThrowsAsync<WorkerFrameProtocolException>(
+                async () => await writer.WriteAsync(oversized));
+        Assert.Equal(WorkerFrameProtocolErrorCode.MessageTooLarge, exception.ErrorCode);
+
+        await writer.WriteAsync(CreateEventEnvelope());
+
+        stream.Position = 0;
+        WorkerFrameReader reader = new(stream, options);
+        WorkerEnvelope first = await reader.ReadAsync();
+        WorkerEnvelope second = await reader.ReadAsync();
+
+        // Two frames reached the wire; the rejected frame in between left no gap.
+        Assert.Equal(1UL, first.Sequence);
+        Assert.Equal(2UL, second.Sequence);
+        Assert.Equal(stream.Length, stream.Position);
+    }
+
     /// <summary>Verifies a zero negotiated frame maximum keeps the constructor default.</summary>
     [Fact]
     public void AdoptNegotiatedMaxMessageBytes_WithZero_KeepsDefault()