fix(siteruntime): guard the DCL arbitration overlay against stale store snapshots — register row 101

An off-thread snapshot read racing an artifact apply could land after the
apply populated the overlay; its stale absences then tombstoned and stopped
just-applied LIVE connections, which nothing re-creates until an instance
redeploy (the removal evicts the hash slot the overlay push is gated on).
Each read now captures a definitions-write generation at dispatch; a stale
result is discarded whole (additions too — a stale row would reconfigure a
live connection backward). Deterministic repro fails before the fix; a
fresh-generation guard test pins that row-90 disappearance still fires.

tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/DclConnectionArbitrationTests.cs

@@ -567,6 +567,69 @@ public class DclConnectionArbitrationTests : TestKit, IDisposable
         Assert.Equal(reAddedEndpoint, EndpointOf(CreatesFor(ConnectionName).Last()));
     }
 
+    /// <summary>
+    /// Deferred register row 101 — the stale-snapshot race, made deterministic. A sweep's store
+    /// read runs OFF the actor thread; one that starts before an artifact apply's store writes
+    /// can land AFTER <c>HandleApplyArtifactDataConnectionsToDcl</c> populated the overlay. Its
+    /// snapshot then lacks the just-applied rows, and the disappearance detection would tombstone
+    /// and STOP the just-created live connections — which nothing re-creates until an instance
+    /// redeploy, because the removal also evicts the hash slot the overlay's push is gated on.
+    ///
+    /// <para>
+    /// The crafted <c>InstanceSweepLoaded</c> below IS that race: an empty snapshot stamped with
+    /// the pre-apply write generation, delivered after the apply. <b>Fails before the fix</b>:
+    /// without the generation guard the actor removes the connection it created a moment ago.
+    /// </para>
+    /// </summary>
+    [Fact]
+    public async Task StaleSweepSnapshot_RacingAnArtifactApply_DoesNotRemoveJustAppliedConnections()
+    {
+        // Sweep effectively disabled so the crafted message is the only sweep result in play.
+        var dm = CreateBootedDeploymentManager(sweepSeconds: 3600);
+
+        dm.Tell(new DeployArtifactsCommand(
+            "artifact-race-1", null, null, null, null,
+            [new DataConnectionArtifact(ConnectionName, "OpcUa", EndpointJson(DefinitionEndpoint), null)],
+            null, DateTimeOffset.UtcNow));
+        Assert.True(ExpectMsg<ArtifactDeploymentResponse>(TimeSpan.FromSeconds(20)).Success);
+        await AwaitCreateCountAsync(1, "the artifact apply never created the connection");
+
+        // Generation 0 = captured before the apply bumped it; empty list = the pre-write table.
+        dm.Tell(new DeploymentManagerActor.InstanceSweepLoaded(
+            [], null, ConnectionDefinitions: [], DefinitionsGenerationAtRead: 0));
+
+        await Task.Delay(TimeSpan.FromSeconds(2));
+        Assert.DoesNotContain(_removes, r => r.ConnectionName == ConnectionName);
+    }
+
+    /// <summary>
+    /// The row-101 guard must not blunt row 90's legitimate disappearance detection: the same
+    /// empty snapshot stamped with the CURRENT write generation is a real answer (the store
+    /// genuinely holds no rows any more) and still stops the connection.
+    /// </summary>
+    [Fact]
+    public async Task FreshEmptySweepSnapshot_StillRemovesDisappearedConnections()
+    {
+        var dm = CreateBootedDeploymentManager(sweepSeconds: 3600);
+
+        dm.Tell(new DeployArtifactsCommand(
+            "artifact-race-2", null, null, null, null,
+            [new DataConnectionArtifact(ConnectionName, "OpcUa", EndpointJson(DefinitionEndpoint), null)],
+            null, DateTimeOffset.UtcNow));
+        Assert.True(ExpectMsg<ArtifactDeploymentResponse>(TimeSpan.FromSeconds(20)).Success);
+        await AwaitCreateCountAsync(1, "the artifact apply never created the connection");
+
+        // Exactly one artifact apply has run, so the current write generation is 1.
+        dm.Tell(new DeploymentManagerActor.InstanceSweepLoaded(
+            [], null, ConnectionDefinitions: [], DefinitionsGenerationAtRead: 1));
+
+        await AwaitAssertAsync(() =>
+        {
+            Assert.Contains(_removes, r => r.ConnectionName == ConnectionName);
+            return Task.CompletedTask;
+        }, TimeSpan.FromSeconds(10));
+    }
+
     /// <summary>
     /// Stand-in for the DCL manager actor: records the two commands the Deployment Manager is the
     /// sole sender of, and swallows everything else (Instance Actors send their tag subscribes