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/MxAccess/MxAccessEventQueueTests.cs

@@ -29,6 +29,26 @@ public sealed class MxAccessEventQueueTests
         Assert.False(queue.TryDequeue(out _));
     }
 
+    /// <summary>
+    ///     Verifies that Enqueue takes ownership of the passed event instead of
+    ///     cloning it: the dequeued instance is the very reference passed in, and
+    ///     the worker sequence/timestamp are stamped on that same instance
+    ///     (WRK-11).
+    /// </summary>
+    [Fact]
+    public void Enqueue_TakesOwnershipOfPassedEventInstance()
+    {
+        MxAccessEventQueue queue = new(capacity: 4);
+        MxEvent original = CreateEvent(MxEventFamily.OnDataChange, itemHandle: 10);
+
+        queue.Enqueue(original);
+
+        Assert.True(queue.TryDequeue(out WorkerEvent? dequeued));
+        Assert.Same(original, dequeued?.Event);
+        Assert.Equal(1UL, original.WorkerSequence);
+        Assert.NotNull(original.WorkerTimestamp);
+    }
+
     /// <summary>Verifies that Drain removes at most the requested number of events.</summary>
     [Fact]
     public void Drain_RemovesAtMostRequestedEvents()

src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/MxAccessValueCacheTests.cs

@@ -46,6 +46,37 @@ public sealed class MxAccessValueCacheTests
         Assert.Equal(999, other.Value.Int32Value);
     }
 
+    /// <summary>
+    ///     Verifies that Set stores an independent deep-copied snapshot: mutating
+    ///     the source event's protobuf sub-messages after caching does not alter
+    ///     the cached value. WRK-11 stopped the event sink cloning before enqueue,
+    ///     so the same MxEvent instance now flows to the outbound queue; the cache
+    ///     must own its own copy so the two never share mutable state.
+    /// </summary>
+    [Fact]
+    public void Set_StoresIndependentSnapshot_UnaffectedByLaterEventMutation()
+    {
+        MxAccessValueCache cache = new();
+        Timestamp sourceTimestamp = Timestamp.FromDateTime(new(2026, 5, 19, 9, 0, 0, DateTimeKind.Utc));
+        MxEvent mxEvent = BuildEvent(serverHandle: 7, itemHandle: 21, intValue: 100, quality: 192, sourceTimestamp);
+
+        cache.Set(7, 21, mxEvent);
+
+        // Mutate the event in place after it was cached — as if it kept flowing
+        // through the (unrelated) outbound path. None of this must reach the cache.
+        mxEvent.Value.Int32Value = 999;
+        mxEvent.Quality = 0;
+        mxEvent.SourceTimestamp = Timestamp.FromDateTime(new(2030, 1, 1, 0, 0, 0, DateTimeKind.Utc));
+        mxEvent.Statuses[0].Category = MxStatusCategory.SecurityError;
+
+        Assert.True(cache.TryGet(7, 21, out MxAccessValueCache.CachedValue cached));
+        Assert.Equal(100, cached.Value.Int32Value);
+        Assert.Equal(192, cached.Quality);
+        Assert.Equal(sourceTimestamp, cached.SourceTimestamp);
+        Assert.Single(cached.Statuses);
+        Assert.Equal(MxStatusCategory.Ok, cached.Statuses[0].Category);
+    }
+
     /// <summary>Verifies that TryGet returns false for unknown handles.</summary>
     [Fact]
     public void TryGet_WithUnknownHandle_ReturnsFalse()