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/MxGatewaySession.cs

@@ -875,6 +875,34 @@ public sealed class MxGatewaySession : IAsyncDisposable
             cancellationToken);
     }
 
+    /// <summary>
+    /// Streams events as typed <see cref="MxEventStreamItem"/> values, surfacing
+    /// the gateway's reconnect-replay gap sentinel as an observable, typed signal.
+    /// </summary>
+    /// <remarks>
+    /// When resuming with a stale <paramref name="afterWorkerSequence"/> (older than
+    /// the oldest event still retained in the session replay ring), the gateway emits
+    /// a single gap sentinel at the head of the stream. It arrives here as an item
+    /// with <see cref="MxEventStreamItem.IsReplayGap"/> true and
+    /// <see cref="MxEventStreamItem.ReplayGap"/> populated, meaning the consumer
+    /// missed events and MUST discard local state and re-snapshot. To resume without
+    /// incurring another gap, reconnect with
+    /// <c>afterWorkerSequence = item.ReplayGap.OldestAvailableSequence - 1</c>.
+    /// All other events pass through with <see cref="MxEventStreamItem.IsReplayGap"/>
+    /// false. Use <see cref="StreamEventsAsync"/> when raw generated
+    /// <see cref="MxEvent"/> messages are needed instead.
+    /// </remarks>
+    /// <param name="afterWorkerSequence">The sequence number to stream from. Defaults to 0.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>An async enumerable of typed event items.</returns>
+    public IAsyncEnumerable<MxEventStreamItem> StreamEventItemsAsync(
+        ulong afterWorkerSequence = 0,
+        CancellationToken cancellationToken = default)
+    {
+        return StreamEventsAsync(afterWorkerSequence, cancellationToken)
+            .AsStreamItemsAsync(cancellationToken);
+    }
+
     /// <summary>
     /// Closes the session and releases resources.
     /// </summary>