feat(alarms): feed-level snapshot_status truncation frame on StreamAlarms

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

@@ -1018,6 +1018,10 @@ message AlarmFeedMessage {
     // Provider-mode status. Emitted once on stream open and again on every
     // failover/failback so late joiners learn the current mode immediately.
     AlarmProviderStatus provider_status = 4;
+    // Snapshot-completeness status. Emitted once on stream open and again on
+    // every change of the truncation verdict, so late joiners learn whether the
+    // feed's active-alarm set may be incomplete.
+    AlarmSnapshotStatus snapshot_status = 5;
   }
 }
 
@@ -1028,6 +1032,20 @@ message AlarmProviderStatus {
   google.protobuf.Timestamp since = 4;
 }
 
+// Feed-level snapshot-completeness status. Emitted once on StreamAlarms open
+// (after the initial provider_status frame, before the cached active_alarm
+// frames) so late joiners learn the current verdict, and again on every change
+// of the truncation verdict observed at reconcile. Mirrors the per-record
+// ActiveAlarmSnapshot.from_truncated_snapshot caveat at feed level so live
+// consumers can reason about completeness without polling QueryActiveAlarms.
+message AlarmSnapshotStatus {
+  // True while the monitor's cached active-alarm set derives from a truncated
+  // (capped) worker fetch — the set may be missing alarms. Distinct from
+  // provider degradation (AlarmProviderStatus.degraded), which describes the
+  // fidelity of the records rather than the completeness of the set.
+  bool truncated = 1;
+}
+
 message MxStatusProxy {
   // Mirrors the `success` member of the MXAccess MXSTATUS_PROXY struct
   // (a 16-bit signed value in the COM struct, widened to int32 on the