Resolve all 33 medium-severity architecture review findings

Second slice of the 2026-09-09 whole-system review remediation
(docs/plans/2026-09-09-arch-review.md §10), on top of the High slice 146a1323.

Runtime/devices: SR-2 alarm on-trigger integration scope; SR-3 correlated NativeAlarmActor
subscribe retry; SR-4 immutable audited DB target; SR-5 ordered InstanceActor stop/flush;
SR-12 rehydration fence; SR-13 S&F gate recheck per delivery; DC-4 no OPC UA security
downgrade; DC-5 alarm snapshot for every new subscriber; ORCH-1 sync-dispose joins the pump.

Management/UI: MG-2 site scope on audit tree + Flow Debugger Replay; MG-3 stale-probe memo
removed; MG-4 server-owned bulk-deploy deadline; UI-5 sandbox runs in a killable child
process (new CentralUI.SandboxWorker project); UI-14 bounded sandbox output/IPC.

Edge/audit: EI-1 relay audit ownership; EI-2 SkipBodyCapture suppresses error text; EI-3/EI-11
error-field redaction + caps; EI-4 single relay deadline, pre-execution-only retry; EI-5
authenticated relay provenance; EI-10 body cap before audit read.

Persistence/contracts: DL-1 global Instance.UniqueName unique index + conflict translation;
DL-2 DataConnection protocol config encrypted at rest (migration
20260909203046_EnforceGlobalInstanceNamesAndProtectProtocolConfiguration — coordinated
central upgrade, old binaries cannot read protected rows); DL-3 bounded Replay paging;
DL-6 cross-site Overwrite of a deployed instance rejected; XC-4 proto descriptor contract test.

Config/hygiene: XC-1 central listener isolation by local port; XC-2/XC-3 inert SMTP + cron
keys removed from rig settings; CM-2 env2 bootstrap guard enabled; DC-6 deploy site exit code;
DC-7 notifier retries only explicit standby rejection; XC-5 ClusterClient purged from docs.

Gates recorded in the report: build 0/0, 9 397 passed / 0 failed, rig redeployed on all
eight nodes with the migration applied. 46 Low findings remain open.

src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Adapters/MxGatewayDataConnection.cs

@@ -319,8 +319,8 @@ public class MxGatewayDataConnection
             // MxGateway has no server-side CONDITION filter either, so conditionFilter is
             // still not forwarded: the DataConnectionActor's per-source + per-condition-type
             // gate stays authoritative, uniform with the OPC UA WhereClause stance.
-            if (_alarmCts != null && AlarmFilterPrefix.Covers(_alarmStreamPrefix, sourceReference))
-                return Task.FromResult(subscriptionId);
+            // Even a covered new source needs a standing-state snapshot. Reopen
+            // the shared stream; DCL separately replays same-source co-subscribers.
 
             // A new source outside the live prefix (or the first subscriber): (re)open the
             // stream on the recomputed union. The source replays a fresh snapshot, which
@@ -336,7 +336,23 @@ public class MxGatewayDataConnection
             var token = _alarmCts.Token;
             var client = _client!;
             var prefix = _alarmStreamPrefix.Length == 0 ? null : _alarmStreamPrefix;
-            _ = Task.Run(() => client.RunAlarmStreamAsync(prefix, t => callback(t), token), token);
+            var epoch = Guid.NewGuid();
+            void Forward(Commons.Types.Alarms.NativeAlarmTransition transition)
+            {
+                lock (_alarmLock)
+                {
+                    if (token.IsCancellationRequested) return;
+                    if (transition.Kind == AlarmTransitionKind.SnapshotStart) epoch = Guid.NewGuid();
+                    callback(transition.Kind is AlarmTransitionKind.SnapshotStart or AlarmTransitionKind.Snapshot or AlarmTransitionKind.SnapshotComplete
+                        ? transition with { SnapshotId = epoch } : transition);
+                }
+            }
+            _ = Task.Run(async () =>
+            {
+                Forward(MxGatewayAlarmMapper.SnapshotComplete() with { Kind = AlarmTransitionKind.SnapshotStart });
+                if (token.IsCancellationRequested) return;
+                await client.RunAlarmStreamAsync(prefix, Forward, token);
+            }, token);
         }
 
         return Task.FromResult(subscriptionId);