perf(worker): event hot-path allocation + flush cuts (WRK-06/11/12, IPC-15)

WRK-06: MxStatusProxyConverter caches the four resolved FieldInfo per status
type in a static ConcurrentDictionary (the GetField metadata scan ran 4x per
status per event on the STA path). GetValue+Convert.ToInt32 still run per event
(late-bound RCW). Exceptions byte-identical: missing-field message unchanged
(ResolveField, not cached on throw via GetOrAdd); null-value message unchanged.

WRK-11: MxAccessEventQueue.Enqueue takes ownership of the passed MxEvent -
stamps WorkerSequence/WorkerTimestamp on it in place and enqueues it, no
Clone(). Audited all 3 callers (base/alarm event sinks, provider-mode handler):
each builds a fresh event per Enqueue, none reuse it. MxAccessValueCache.Set now
deep-copies its retained Value/SourceTimestamp/Statuses so the cache snapshot
never aliases the queue-owned (later serialized) event. Net: alarm/other events
clone nothing (was full clone); data-change clones payload-only.

WRK-12: WorkerFrameWriter coalesces the flush across a drained batch - each
frame is written but not flushed individually; one FlushAsync after the batch,
then all written frames complete. Preserves the written+flushed completion
contract; a burst of N events costs 1 flush, not N. On write failure the whole
in-flight batch + queue fail so no caller hangs.

IPC-15 (doc): the multi-event WorkerEnvelope body remains unimplemented (wire
still carries one event per worker_event frame); gateway.md Performance section
now distinguishes the shipped flush-coalescing from that deferred proto change.

net48-safe (no init/records; readonly struct cache entry). Worker builds x86
only - verification on windev. Tests added: converter cache-reuse, queue
ownership-transfer, value-cache snapshot independence, writer batch-flush-once.

Claude-Session: https://claude.ai/code/session_01DMXXvNuPekkkrTEyPNxEkW

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

@@ -373,6 +373,43 @@ public sealed class WorkerFrameProtocolTests
         Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerEvent, frame3.BodyCase);
     }
 
+    /// <summary>
+    /// Verifies the writer coalesces the flush across a batch of frames drained together: four frames
+    /// queued behind an in-progress write drain in a single pass and share one FlushAsync, not four
+    /// (WRK-12 / IPC-15). Every frame still reaches the wire intact.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task WriteAsync_WhenBatchDrainedTogether_FlushesOnce()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        using GatedWriteStream stream = new();
+        WorkerFrameWriter writer = new(stream, options);
+
+        // A blocked first write occupies the writer and holds the lock while more frames queue behind it.
+        Task firstWrite = writer.WriteAsync(CreateGatewayHelloEnvelope(), WorkerFrameWritePriority.Control);
+        await AwaitWithTimeoutAsync(stream.FirstWriteStarted);
+
+        Task eventWrite1 = writer.WriteAsync(CreateEventEnvelope(), WorkerFrameWritePriority.Event);
+        Task eventWrite2 = writer.WriteAsync(CreateEventEnvelope(), WorkerFrameWritePriority.Event);
+        Task eventWrite3 = writer.WriteAsync(CreateEventEnvelope(), WorkerFrameWritePriority.Event);
+        await Task.Delay(50);
+
+        stream.ReleaseFirstWrite();
+        await AwaitWithTimeoutAsync(Task.WhenAll(firstWrite, eventWrite1, eventWrite2, eventWrite3));
+
+        // Four frames written in one drain pass => exactly one flush.
+        Assert.Equal(1, stream.FlushCount);
+
+        stream.Position = 0;
+        WorkerFrameReader reader = new(stream, options);
+        for (int index = 0; index < 4; index++)
+        {
+            WorkerEnvelope frame = await reader.ReadAsync();
+            Assert.NotEqual(WorkerEnvelope.BodyOneofCase.None, frame.BodyCase);
+        }
+    }
+
     /// <summary>Verifies a zero negotiated frame maximum keeps the constructor default (IPC-02).</summary>
     [Fact]
     public void AdoptNegotiatedMaxMessageBytes_WithZero_KeepsDefault()
@@ -451,9 +488,12 @@ public sealed class WorkerFrameProtocolTests
         private readonly TaskCompletionSource<bool> _firstWriteStarted =
             new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
         private int _writeCount;
+        private int _flushCount;
 
         public Task FirstWriteStarted => _firstWriteStarted.Task;
 
+        public int FlushCount => Volatile.Read(ref _flushCount);
+
         public void ReleaseFirstWrite() => _release.Release();
 
         public override async Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
@@ -467,6 +507,12 @@ public sealed class WorkerFrameProtocolTests
             await base.WriteAsync(buffer, offset, count, cancellationToken);
         }
 
+        public override Task FlushAsync(CancellationToken cancellationToken)
+        {
+            Interlocked.Increment(ref _flushCount);
+            return base.FlushAsync(cancellationToken);
+        }
+
         protected override void Dispose(bool disposing)
         {
             if (disposing)