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.SiteRuntime/Scripts/AuditingDbConnection.cs

@@ -111,14 +111,13 @@ internal sealed class AuditingDbConnection : DbConnection
     internal DbConnection Inner => _inner;
 
     /// <inheritdoc />
-    // ConnectionString is settable on DbConnection — forward both halves.
+    // The configured target is the audit identity; scripts cannot retarget it or
+    // retrieve connection credentials through the raw ADO surface.
     public override string ConnectionString
     {
-        // Some providers throw on get when the connection hasn't been opened
-        // with a string set explicitly. The wrapper has no opinion — forward.
-#pragma warning disable CS8765 // nullability of overridden member parameter — base setter accepts null in practice
-        get => _inner.ConnectionString;
-        set => _inner.ConnectionString = value;
+#pragma warning disable CS8765
+        get => string.Empty;
+        set => throw new NotSupportedException("Audited connections cannot be retargeted.");
 #pragma warning restore CS8765
     }
 
@@ -135,7 +134,7 @@ internal sealed class AuditingDbConnection : DbConnection
     public override ConnectionState State => _inner.State;
 
     /// <inheritdoc />
-    public override void ChangeDatabase(string databaseName) => _inner.ChangeDatabase(databaseName);
+    public override void ChangeDatabase(string databaseName) => throw new NotSupportedException("Audited connections cannot be retargeted.");
 
     /// <inheritdoc />
     public override void Close() => _inner.Close();