fix(data-connection-layer): resolve DataConnectionLayer-002/003/004/005 — Resume supervision, concurrent dicts, subscribe-failure classification, write timeout

src/ScadaLink.DataConnectionLayer/Actors/DataConnectionManagerActor.cs

@@ -125,8 +125,20 @@ public class DataConnectionManagerActor : ReceiveActor
     }
 
     /// <summary>
-    /// OneForOneStrategy with Restart for connection actors — a failed connection
-    /// should restart and attempt reconnection.
+    /// OneForOneStrategy with Resume for connection actors.
+    ///
+    /// DataConnectionLayer-002: a DataConnectionActor is a long-lived, stateful
+    /// coordinator — its in-memory subscription registry (_subscriptionsByInstance,
+    /// _subscriptionIds, _subscribers) is the only record of which Instance Actors
+    /// subscribed to which tags, and there is no durable store to rebuild it from.
+    /// Restart would create a fresh instance and silently discard that registry,
+    /// breaking the design doc's "transparent re-subscribe" guarantee (WP-10):
+    /// subscribers would never be re-subscribed and would sit at stale quality with
+    /// no error. Resume keeps the actor instance and its state intact, so a transient
+    /// exception in a message handler does not lose subscription state. The actor's
+    /// own Become/Stash reconnect state machine already recovers connection-level
+    /// faults, so it does not need a restart to re-establish the connection.
+    /// This matches the ScadaLink convention of Resume for coordinator actors.
     /// </summary>
     protected override SupervisorStrategy SupervisorStrategy()
     {
@@ -135,8 +147,8 @@ public class DataConnectionManagerActor : ReceiveActor
             withinTimeRange: TimeSpan.FromMinutes(1),
             decider: Decider.From(ex =>
             {
-                _log.Warning(ex, "DataConnectionActor threw exception, restarting");
-                return Directive.Restart;
+                _log.Warning(ex, "DataConnectionActor threw exception, resuming (subscription state preserved)");
+                return Directive.Resume;
             }));
     }
 }