fix(opcua): #855 review 4 — per-item "known" in the folder cut; pin the post-stall liveness check

- RefreshDirectItemsWithCut keys its "decided off Lock" set per (item,
  condition), so a condition one item's walk saw is still decided for another.
- The deferral warning no longer blames slow folder subscribers (no folder of
  ours defers since #855).
- FolderEventFloodWireTests' removal test now asserts the folder item got
  nothing and the Server object exactly the pre-stall event (it passed with
  the post-stall liveness re-check deleted).

Claude-Session: https://claude.ai/code/session_01GT67AD99zQQqpLCVm3p7xk

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests/FolderEventFloodWireTests.cs

@@ -193,7 +193,10 @@ public sealed class FolderEventFloodWireTests
         Flooded(onServer).ShouldNotBeEmpty();
         // None of the deferred tail — which was still waiting when the condition was removed — reached anyone.
         Flooded(onServer).Max().ShouldBeLessThan(FloodSize - 1, "no ghost reached the Server-object subscriber");
-        Flooded(onA).DefaultIfEmpty(-1).Max().ShouldBeLessThan(FloodSize - 1, "no ghost reached the folder subscriber");
+        // Only the in-flight f0 ever reached the stalled EqA hop, and the condition was gone when the stall released: the
+        // post-stall liveness re-check must have withheld it (#855 review 4 — "< FloodSize - 1" passed without that check).
+        Flooded(onA).ShouldBeEmpty("no ghost reached the folder subscriber");
+        Flooded(onServer).ShouldBe([0], "only the event reported before the stall reached the Server object");
     }
 
     private static AlarmConditionSnapshot State(string message) =>