feat(clients): CLI-15 surface ReplayGap reconnect sentinel as typed signal (4/5 clients)

The gateway emits a ReplayGap sentinel MxEvent at the head of a StreamEvents
stream resumed via after_worker_sequence when the requested cursor predates the
oldest retained event. Clients previously ignored it, silently mis-treating a
lossy resume as continuous. Each client now surfaces the sentinel as a distinct,
typed, non-terminal signal (never synthesized, never swallowed) so a consumer can
detect the gap and re-snapshot; resume contract is
after_worker_sequence = oldest_available_sequence - 1.

- .NET: MxEventStreamItem (IsReplayGap/ReplayGap/Event) via new StreamEventItemsAsync
  + AsStreamItemsAsync extension. Build clean, 87 passed.
- Go: EventResult.ReplayGap field + IsReplayGap(); ReplayGap type alias. build/vet/test clean.
- Rust: EventItem enum (Event/ReplayGap); EventStream now yields Result<EventItem, Error>;
  CLI renders REPLAY_GAP line / replayGap JSON. fmt/check/test/clippy clean.
- Python: ReplayGap dataclass; stream_events yields pb.MxEvent | ReplayGap. 131 passed.
- Shared docs: ClientLibrariesDesign non-goals reframed (reconnect-replay protocol is
  consumable; auto-reconnect stays a non-goal); CrossLanguageSmokeMatrix resume-gap note.

Java client is deferred to the windev batch (no local JRE); CLI-15 stays open until it lands.

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

clients/dotnet/ZB.MOM.WW.MxGateway.Client/MxEventStreamExtensions.cs

@@ -0,0 +1,40 @@
+using System.Runtime.CompilerServices;
+using ZB.MOM.WW.MxGateway.Contracts.Proto;
+
+namespace ZB.MOM.WW.MxGateway.Client;
+
+/// <summary>
+/// Extension methods that project a raw <see cref="MxEvent"/> stream into the
+/// typed <see cref="MxEventStreamItem"/> surface, making the gateway's
+/// reconnect-replay gap sentinel observable.
+/// </summary>
+public static class MxEventStreamExtensions
+{
+    /// <summary>
+    /// Projects a raw <see cref="MxEvent"/> stream (e.g.
+    /// <see cref="MxGatewaySession.StreamEventsAsync"/> or
+    /// <see cref="MxGatewayClient.StreamEventsAsync"/>) into typed
+    /// <see cref="MxEventStreamItem"/> values. Normal events pass through with
+    /// <see cref="MxEventStreamItem.IsReplayGap"/> false; the gateway's
+    /// reconnect-replay gap sentinel is surfaced with
+    /// <see cref="MxEventStreamItem.IsReplayGap"/> true and
+    /// <see cref="MxEventStreamItem.ReplayGap"/> populated. The stream is
+    /// forwarded faithfully — no event is synthesized or dropped.
+    /// </summary>
+    /// <param name="source">The raw event stream to wrap.</param>
+    /// <param name="cancellationToken">Cancellation token for the enumeration.</param>
+    /// <returns>The same events, each wrapped as an <see cref="MxEventStreamItem"/>.</returns>
+    public static async IAsyncEnumerable<MxEventStreamItem> AsStreamItemsAsync(
+        this IAsyncEnumerable<MxEvent> source,
+        [EnumeratorCancellation] CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(source);
+
+        await foreach (MxEvent gatewayEvent in source
+            .WithCancellation(cancellationToken)
+            .ConfigureAwait(false))
+        {
+            yield return MxEventStreamItem.From(gatewayEvent);
+        }
+    }
+}