feat(contracts): add ReplayGap signal to MxEvent for reconnect replay

src/ZB.MOM.WW.MxGateway.Contracts/Protos/mxaccess_gateway.proto

@@ -715,6 +715,20 @@ message MxEvent {
   google.protobuf.Timestamp gateway_receive_timestamp = 11;
   optional int32 hresult = 12;
   string raw_status = 13;
+  // Gateway-synthesized reconnect-replay gap signal. Set ONLY on the single
+  // sentinel MxEvent the gateway emits at the head of a StreamEvents stream
+  // that was resumed via StreamEventsRequest.after_worker_sequence when the
+  // requested sequence is older than the oldest event still retained in the
+  // session replay ring (i.e. events were evicted and cannot be replayed).
+  // On that sentinel, `family` is UNSPECIFIED, the `body` oneof is unset, and
+  // no per-item fields (server_handle/item_handle/value/...) are populated;
+  // clients MUST treat a present `replay_gap` as "you missed events — discard
+  // local state and re-snapshot" and read `requested_after_sequence` /
+  // `oldest_available_sequence` from it. Unset on every normal MXAccess event.
+  // Additive (proto3): existing clients that ignore this field continue to
+  // deserialize the stream unchanged. (Reconnect/replay logic is Task 12; this
+  // is the contract surface only.)
+  optional ReplayGap replay_gap = 14;
 
   oneof body {
     OnDataChangeEvent on_data_change = 20;
@@ -726,6 +740,22 @@ message MxEvent {
   }
 }
 
+// Reconnect-replay gap signal carried by a sentinel MxEvent (MxEvent.replay_gap)
+// when a client resumes StreamEvents via after_worker_sequence but the requested
+// sequence predates the oldest event still held in the session replay ring.
+// The events in the open interval (requested_after_sequence, oldest_available_sequence)
+// were evicted from the ring and cannot be replayed, so the client must
+// re-snapshot rather than assume a contiguous event history.
+message ReplayGap {
+  // The worker_sequence the client asked to resume after
+  // (StreamEventsRequest.after_worker_sequence).
+  uint64 requested_after_sequence = 1;
+  // The oldest worker_sequence still retained in the replay ring. Events with
+  // worker_sequence in (requested_after_sequence, oldest_available_sequence)
+  // were evicted and are unrecoverable.
+  uint64 oldest_available_sequence = 2;
+}
+
 enum MxEventFamily {
   MX_EVENT_FAMILY_UNSPECIFIED = 0;
   MX_EVENT_FAMILY_ON_DATA_CHANGE = 1;