fix(mtconnect): on a changed alarm set forget only the conditions that left (#641)

The driver cleared its whole "last told" record whenever the subscribed alarm
set changed, assuming the rebuild that accompanies an alarm-bearing delta had
reset every condition node. The server now carries each surviving native
condition's state (acknowledgement included) across the rebuild, so re-raising a
survivor would un-acknowledge it. Only entries that left the set are dropped; a
condition that comes back later is told again onto its fresh node.

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDriver.Alarms.cs

@@ -50,18 +50,13 @@ public sealed partial class MTConnectDriver : IAlarmSource
     /// <summary>
     ///     ConditionId (RawPath) → what the server was last told. Guarded by <see cref="_alarmLock"/>. Survives a
     ///     re-subscribe with the SAME reference set (a reconnect) and lifecycle restarts — the server's condition
-    ///     nodes outlive both, and re-raising there would un-acknowledge. Cleared WHOLE when the subscribed set
-    ///     changes: only an alarm-bearing raw-tag delta changes it, that delta is a full address-space rebuild,
-    ///     and a rebuild recreates every condition node inactive — so everything must be told again.
+    ///     nodes outlive both, and re-raising there would un-acknowledge. When the subscribed set changes (an
+    ///     alarm-bearing raw-tag delta, i.e. a full rebuild) only the entries that LEFT the set are dropped: the
+    ///     server carries every surviving condition's state across the rebuild (Gitea #641), so a survivor needs
+    ///     no re-tell, while a condition that comes back later was materialised fresh and is told again.
     /// </summary>
     private readonly Dictionary<string, MTConnectToldCondition> _deliveredConditions = new(StringComparer.Ordinal);
 
-    /// <summary>
-    ///     The reference set of the last <see cref="SubscribeAlarmsAsync"/>, kept across an unsubscribe so a
-    ///     release-then-resubscribe of an identical set is still recognised. Guarded by <see cref="_alarmLock"/>.
-    /// </summary>
-    private HashSet<string>? _lastAlarmReferences;
-
     /// <summary>The CONDITION tracker for the current session; replaced alongside the observation index.</summary>
     private MTConnectConditionTracker _conditions = new();
 
@@ -108,16 +103,15 @@ public sealed partial class MTConnectDriver : IAlarmSource
 
         lock (_alarmLock)
         {
-            // A CHANGED set means an alarm-bearing delta, which the server applies as a full rebuild: every
-            // condition node — including ones still in the set — was recreated inactive and has been told
-            // nothing. Re-tell all of them. The SAME set is a reconnect: keep the record, or every still-active
-            // alarm would be re-raised and lose its acknowledgement.
-            if (_lastAlarmReferences is null || !_lastAlarmReferences.SetEquals(references))
+            // Forget only the conditions that LEFT the set (Gitea #641). A changed set is an alarm-bearing delta,
+            // which the server applies as a full rebuild — but the server carries every surviving condition's
+            // state (acknowledgement included) across that rebuild, so re-raising a survivor would only
+            // un-acknowledge it. A condition that left and later comes back was materialised fresh and has no
+            // record, so it is told again. The SAME set (a reconnect) prunes nothing.
+            foreach (var conditionId in _deliveredConditions.Keys.Where(k => !references.Contains(k)).ToList())
             {
-                _deliveredConditions.Clear();
+                _deliveredConditions.Remove(conditionId);
             }
-
-            _lastAlarmReferences = references;
         }
 
         EmitConditionTransitions(touchedDataItemIds: null, resync: true);