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/go/mxgateway/client_session_test.go

@@ -200,6 +200,63 @@ func TestEventsSlowConsumerYieldsErrSlowConsumerBeforeClose(t *testing.T) {
 	}
 }
 
+func TestEventsSurfacesReplayGapSentinelAsTypedSignal(t *testing.T) {
+	fake := &fakeGatewayServer{
+		streamStarted: make(chan struct{}),
+		streamReplayGap: &pb.ReplayGap{
+			RequestedAfterSequence:  5,
+			OldestAvailableSequence: 42,
+		},
+	}
+	client, cleanup := newBufconnClient(t, fake)
+	defer cleanup()
+	session := NewSessionForID(client, "session-1")
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	events, err := session.EventsAfter(ctx, 5)
+	if err != nil {
+		t.Fatalf("EventsAfter() error = %v", err)
+	}
+	<-fake.streamStarted
+
+	// First result must be the typed replay-gap signal, not a normal event.
+	first := <-events
+	if first.Err != nil {
+		t.Fatalf("first result error = %v", first.Err)
+	}
+	if !first.IsReplayGap() {
+		t.Fatalf("first result IsReplayGap() = false, want true")
+	}
+	if first.Event != nil {
+		t.Fatalf("replay-gap result carried a non-nil Event %+v; want the sentinel to clear Event", first.Event)
+	}
+	if got := first.ReplayGap.GetRequestedAfterSequence(); got != 5 {
+		t.Fatalf("ReplayGap.RequestedAfterSequence = %d, want 5", got)
+	}
+	if got := first.ReplayGap.GetOldestAvailableSequence(); got != 42 {
+		t.Fatalf("ReplayGap.OldestAvailableSequence = %d, want 42", got)
+	}
+
+	// Normal events after the sentinel are unaffected: Event set, ReplayGap nil.
+	second := <-events
+	if second.Err != nil {
+		t.Fatalf("second result error = %v", second.Err)
+	}
+	if second.IsReplayGap() {
+		t.Fatalf("second result IsReplayGap() = true, want false for a normal event")
+	}
+	if second.Event == nil {
+		t.Fatal("second result Event = nil, want a normal event")
+	}
+	if got := second.Event.GetWorkerSequence(); got != 1 {
+		t.Fatalf("normal event worker sequence = %d, want 1", got)
+	}
+	if second.Event.GetFamily() != pb.MxEventFamily_MX_EVENT_FAMILY_ON_DATA_CHANGE {
+		t.Fatalf("normal event family = %s, want ON_DATA_CHANGE", second.Event.GetFamily())
+	}
+}
+
 func TestSessionHelpersBuildCommandsAndExposeRawReply(t *testing.T) {
 	fake := &fakeGatewayServer{
 		invokeReply: &pb.MxCommandReply{
@@ -643,6 +700,7 @@ type fakeGatewayServer struct {
 	streamStarted    chan struct{}
 	streamDone       chan struct{}
 	streamEventCount int
+	streamReplayGap  *pb.ReplayGap
 	invokeReply      *pb.MxCommandReply
 	invokeRequest    *pb.MxCommandRequest
 }
@@ -691,6 +749,16 @@ func (s *fakeGatewayServer) StreamEvents(req *pb.StreamEventsRequest, stream grp
 	if s.streamStarted != nil {
 		close(s.streamStarted)
 	}
+	if s.streamReplayGap != nil {
+		// Emit the reconnect-replay gap sentinel at the head of the resumed
+		// stream: family UNSPECIFIED, body unset, only replay_gap populated.
+		if err := stream.Send(&pb.MxEvent{
+			SessionId: req.GetSessionId(),
+			ReplayGap: s.streamReplayGap,
+		}); err != nil {
+			return err
+		}
+	}
 	eventCount := s.streamEventCount
 	if eventCount == 0 {
 		eventCount = 1