fix(dcl): deliver initial-read seed value after subscription registration

DataConnectionActor seeded a tag's initial value by Tell-ing TagValueReceived
from HandleSubscribe's background task, which runs BEFORE HandleSubscribeCompleted
registers the instance's tags in _subscriptionsByInstance. HandleTagValueReceived's
fan-out then found no subscriber and dropped the value. A tag that soon gets a
data-change notification recovers, but a STATIC tag (e.g. an idle MES field that
never changes) was left Uncertain forever — the dropped seed was its only value.

Seeds now ride back on SubscribeCompleted and are delivered after registration,
reusing HandleTagValueReceived's generation guard, fan-out and quality accounting.
+1 regression test (DCL026).

tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/DataConnectionActorTests.cs

@@ -546,6 +546,43 @@ public class DataConnectionActorTests : TestKit
         Assert.True(ack.Success);
     }
 
+    [Fact]
+    public async Task DCL026_StaticTagSeedValue_IsDeliveredAfterRegistration()
+    {
+        // Regression test for DataConnectionLayer-026. The initial-read seed value used to
+        // be emitted (TagValueReceived) from HandleSubscribe's background task BEFORE
+        // HandleSubscribeCompleted registered the instance's tags in
+        // _subscriptionsByInstance. HandleTagValueReceived's fan-out then found no
+        // subscriber for the tag and silently dropped the value. A tag that soon gets a
+        // real data-change notification recovers, but a STATIC tag (subscribe succeeds,
+        // callback never fires again — e.g. an idle MES field) was left Uncertain forever.
+        // After the fix the seed rides on SubscribeCompleted and is delivered AFTER
+        // registration, so the subscriber receives it.
+        _mockAdapter.ConnectAsync(Arg.Any<IDictionary<string, string>>(), Arg.Any<CancellationToken>())
+            .Returns(Task.CompletedTask);
+        _mockAdapter.Status.Returns(ConnectionHealth.Connected);
+        // Subscribe succeeds; the adapter never invokes the value callback (a static tag).
+        _mockAdapter.SubscribeAsync(Arg.Any<string>(), Arg.Any<SubscriptionCallback>(), Arg.Any<CancellationToken>())
+            .Returns(_ => Task.FromResult("sub-static"));
+        // The gateway returns a Good current value for the static tag.
+        _mockAdapter.ReadAsync(Arg.Any<string>(), Arg.Any<CancellationToken>())
+            .Returns(new ReadResult(true,
+                new TagValue("Left54321", QualityCode.Good, DateTimeOffset.UtcNow), null));
+
+        var actor = CreateConnectionActor("dcl026-static-seed");
+        await Task.Delay(300); // reach Connected state
+
+        actor.Tell(new SubscribeTagsRequest(
+            "c1", "inst1", "dcl026-static-seed",
+            ["MESReceiver_023.MoveInMesContainerNum"], DateTimeOffset.UtcNow));
+
+        // The seeded value must reach the subscriber (was dropped pre-fix).
+        var update = FishForMessage<TagValueUpdate>(_ => true, TimeSpan.FromSeconds(5));
+        Assert.Equal("MESReceiver_023.MoveInMesContainerNum", update.TagPath);
+        Assert.Equal(QualityCode.Good, update.Quality);
+        Assert.Equal("Left54321", update.Value);
+    }
+
     [Fact]
     public async Task DCL004_ConnectionLevelSubscribeFailure_TriggersReconnect_NotTagRetry()
     {