perf(gateway): trim event/command hot-path allocations (GWC-06/07/15, IPC-05)

Behavior-preserving allocation cuts on the per-event/per-command path:
- GWC-06: StreamEvents send-timing uses Stopwatch.GetTimestamp() +
  GetElapsedTime() instead of a per-event Stopwatch allocation (same measured
  span).
- GWC-07/IPC-05 (event): MapEvent transfers ownership of the inner MxEvent
  instead of .Clone()-ing it. Safe: WorkerEvent is parsed fresh per pipe frame
  with the distributor pump as its single consumer (GWC-01), MapEvent runs once
  before fan-out, and every downstream consumer (subscribers, replay ring)
  only READS the event (WorkerSequence is stamped upstream; verified no
  post-mapping mutation). Comment documents the invariant + restore-clone
  caveat if a second consumer is added.
- IPC-05 (command): CreateCommandEnvelope no longer re-clones; MapCommand
  already isolated the graph from the caller-owned gRPC message.
- GWC-15: grpc_stream_queue.depth converts from a per-event push counter to an
  ObservableGauge summing registered channel sources at scrape time only
  (name/semantics unchanged); removes all per-event .Count/lock work.

Kept every load-bearing isolation clone (MapCommand, Invoke, bulk filters,
MapCommandReply). Server build clean (0 warnings); EventStream/Metrics/
Distributor/Mapper tests 62/62 (incl. formerly-flaky queue-depth tests, now
green under the lazy gauge, + 2 new MapEvent ownership tests). Docs: Metrics.md,
Grpc.md.

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

src/ZB.MOM.WW.MxGateway.Server/Grpc/MxAccessGatewayService.cs

@@ -152,9 +152,15 @@ public sealed class MxAccessGatewayService(
                 .WithCancellation(context.CancellationToken)
                 .ConfigureAwait(false))
             {
-                Stopwatch stopwatch = Stopwatch.StartNew();
+                // GWC-06: measure send latency with the allocation-free timestamp API rather
+                // than allocating a Stopwatch object per event per subscriber on the highest-
+                // volume gateway path. Stopwatch.GetTimestamp/GetElapsedTime measure the exact
+                // same wall-clock span the former Stopwatch.StartNew()/.Elapsed did.
+                long sendStartTimestamp = Stopwatch.GetTimestamp();
                 await responseStream.WriteAsync(publicEvent).ConfigureAwait(false);
-                metrics.RecordEventStreamSend(publicEvent.Family.ToString(), stopwatch.Elapsed);
+                metrics.RecordEventStreamSend(
+                    publicEvent.Family.ToString(),
+                    Stopwatch.GetElapsedTime(sendStartTimestamp));
             }
         }
         catch (Exception exception) when (exception is not RpcException)